The airmen started firing at gunners! (3)
I believe the answer is:
raf
'the airmen' is the definition.
(Royal Air Force)
'started firing at gunners' is the wordplay.
'started' says to take the initial letters.
'at' means one lot of letters go next to another (I've seen this in other clues).
'gunners' becomes 'RA' (Royal Artillery).
The initial letter of 'firing' is 'f'.
'f' after 'ra' is 'RAF'.
(Other definitions for raf that I've seen before include "Royal Air Force (abbr.)" , "It fought Battle of Britain" , "Airforce of Great Britain" .)