Adversary of an RAF officer at a loose end (3)
I believe the answer is:
foe
'adversary' is the definition.
(I've seen this before)
'an raf officer at a loose end' is the wordplay.
'an raf officer' becomes 'fo' (flying officer).
'at' says to put letters next to each other.
'end' says to take the final letters.
The final letter of 'loose' is 'e'.
'fo'+'e'='FOE'
'of' acts as a link.
(Other definitions for foe that I've seen before include "friend never!" , "Environmentalists" , "Opponent" , "Someone opposing" , "opposing side" .)