
1) Add baking powder 2) Add yeast 3) Add yeast to 4) Baking powder 5) Cause to rise 6) Do what yeast does 7) French word used in English 8) Get a rise out of 9) Get the batter up 10) Help rise 11) It can raise dough 12) It gets the batter up 13) It makes dough rise 14) Leavening 15) Lightener 16) Make rise as bread 
Found on 
https://www.crosswordclues.com/clue/leaven

1) Barm 2) Cause 3) Component 4) Leavening 5) Yeast 6) Yeastlike 
Found on 
https://www.crosswordclues.com/clue/leaven

• (v. t.) To make light by the action of leaven; to cause to ferment. • (n.) Any substance that produces, or is designed to produce, fermentation, as in dough or liquids; esp., a portion of fermenting dough, which, mixed with a larger quantity of dough, produces a general change in the mass, and renders it light; yeast; barm. • (n.) ...
Found on 
http://thinkexist.com/dictionary/meaning/leaven/

A substance, such as yeast, used to produce fermentation in dough to cause it to rise or lighten. 
Found on 
http://www.encyclo.co.uk/local/20108

Element inducing fermentation. The term is applied to the yeast added to dough in bread-making; it is used figuratively to describe any pervasive influence, usually in a good sense, although in...
Found on 
http://www.encyclo.co.uk/local/20688

The raising agent in dough, usually yeast or baking powder.
Found on 
http://www.encyclo.co.uk/local/21220
 Leav'en  noun
Leav'en  noun [ Middle English 
 levain , 
 levein , French 
 levain , Latin 
 levamen alleviation, mitigation; but taken in the sense of, a raising, that which raises, from 
 levare to raise. See 
 Lever , 
 noun ] 
 1.  Any substance that pro...
Found on 
http://www.encyclo.co.uk/webster/L/25
 Leav'en  transitive verb
Leav'en  transitive verb [ 
 imperfect & past participle   Leavened ; 
 present participle & verbal noun   Leavening .] 
 1.  To make light by the action of leaven; to cause to ferment. « A little leaven 
 leaveneth the whole lump...
Found on 
http://www.encyclo.co.uk/webster/L/25

Leaven refers to dough in which fermentation has commenced, employed to ferment and render light the fresh dough with which it is mingled. Its use dates from remotest antiquity; the addition of yeast or barm being of modern date.
Found on 
http://www.probertencyclopaedia.com/browse/QL.HTM

[
n] - a substance used to produce fermentation in dough or a liquid 2. [n] - an influence that works subtly to lighten or modify something
Found on 
http://www.webdictionary.co.uk/definition.php?query=leaven

substance which is added to dough to produce fermentation; specifically a quantity of fermenting dough reserved from a previous batch to be used for this purpose.
Found on 
http://www2.shu.ac.uk/sfca/glossary.cfm
 prove verb
prove verb cause to puff up with a leaven; `unleavened bread`
Found on 
https://www.encyclo.co.uk/local/20974
 leavening noun
leavening noun an influence that works subtly to lighten or modify something; `his sermons benefited from a leavening of humor`
Found on 
https://www.encyclo.co.uk/local/20974

Element inducing fermentation. The term is applied to the yeast added to dough in bread-making; it is used figuratively to describe any pervasive influence, usually in a good sense, although in the Old Testament it symbolized corruption, and unleavened bread was used in sacrifice
Found on 
https://www.encyclo.co.uk/local/21221

(n.)  also leavening, (v.) include ingredient or mechanical action to leaven or include air space or CO gas in a batter or dough. 
Found on 
https://www.homebaking.org/glossary/
  No exact match found.