
1) Clandestine attackers 2) Secret sympathizers 3) Subversive group of traitors 4) Traitorous group 5) Trojan horse
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https://www.crosswordclues.com/clue/fifth-column

A fifth column is any group of people who undermine a larger group—such as a nation or a besieged city—from within. The activities of a fifth column can be overt or clandestine. Forces gathered in secret can mobilize openly to assist an external attack. This term is also extended to organized actions by military personnel. Clandestine fifth co...
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http://en.wikipedia.org/wiki/Fifth_column

clandestine group or faction of subversive agents who attempt to undermine a nation`s solidarity by any means at their disposal. The term is ...
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http://www.britannica.com/eb/a-z/f/23

Group within a country secretly aiding an enemy attacking from without. The term originated in 1936 during the Spanish Civil War, when General Mola boasted that Franco supporters were attacking...
Found on
http://www.encyclo.co.uk/local/20688

A fifth column is a group within a country secretly aiding an enemy attacking from without. The term originated in 1936 during the Spanish Civil War, when General Mola boasted that Franco supporters were attacking Madrid with four columns and that they had a 'fifth column' inside the city.
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http://www.probertencyclopaedia.com/browse/FF.HTM

[
n] - a subversive group that supports the enemy and engages in espionage or sabotage
Found on
http://www.webdictionary.co.uk/definition.php?query=fifth%20column
Trojan horse noun a subversive group that supports the enemy and engages in espionage or sabotage; an enemy in your midst
Found on
https://www.encyclo.co.uk/local/20974

Group within a country secretly aiding an enemy attacking from without. The term originated in 1936 during the Spanish Civil War, when General Mola boasted that Franco supporters were attacking Madrid with four columns and that they had a `fifth column` inside the city
Found on
https://www.encyclo.co.uk/local/21221

a group of people who act traitorously and subversively out of a secret sympathy with an enemy of their country. · (originally) Franco sympathizers in Madrid during the Spanish Civil War: so called in allusion to a statement in 1936 that the insurgents had four columns marching on Madrid and a fifth column of sympathizers in the city ready ...
Found on
https://www.infoplease.com/dictionary/fifth-column
No exact match found.