Anti-aircraft fire (4)
I believe the answer is:
flak
'anti-aircraft fire' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for flak that I've seen before include "Brickbats" , "that can bring down a plane" , "Unfriendly criticism" , "Criticism (colloq.)" , "heated disagreement" .)