1) Back 2) He lines up behind an end 3) The person who plays wingback 
The word wingback has several senses: ...
(from the article `football (soccer)`) ...marking of forwards by defenders, with the libero providing backup when required. Subsequently, some European clubs introduced 3-5-2 formations ...
[n] - (football) the person who plays wingback 2. [n] - the position of the offensive back who lines up behind or outside the end
noun the position of the offensive back who lines up behind or outside the end