Harsh criticism dangerous to airmen (4)

I believe the answer is:

flak

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'harsh criticism' is the definition.
(I've seen this in another clue)

I don't understand the rest of the clue.

(Other definitions for flak that I've seen before include "complaints" , "Brickbats" , "Ground-to-air attack: verbal abuse" , "Unfriendly criticism" , "Severe criticism" .)

I've seen this clue in the Evening Standard.

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