Exposed a friend to an enemy (8)
I believe the answer is:
betrayed
'exposed a friend to an enemy' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for betrayed that I've seen before include "Unintentionally showed" , "Sold to the enemy" , "Delivered to an enemy by treachery" , "Double-crossed, abandoned" , "given away" .)