Airmen engaged in strafing (3)
I believe the answer is:
raf
'airmen' is the definition.
(Royal Air Force)
'engaged in strafing' is the wordplay.
'engaged in' indicates the answer is hidden within the clue.
'RAF' is hidden in the letters of 'strafing'.
(Other definitions for raf that I've seen before include "It fought Battle of Britain" , "Royal Air Force (abbr.)" , "Airforce of Great Britain" .)