Criticism (4)
'criticism' is the definition.
(I know that flak is a type of criticism)
This is all the clue.
(Other definitions for flak that I've seen before include "Criticism (colloq.)" , "stick" , "that can bring down a plane" , "Ground-to-air attack: verbal abuse" , "Anti-aircraft fire - strong criticism" .)
'criticism' is the definition.
(I know that faultfinding is a type of criticism)
This is all the clue.