Harsh criticism dangerous to airmen (4)

I believe the answer is:

flak

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'harsh criticism' is the definition.
'flak' can be an answer for 'criticism' (I've seen this before). I am not sure about the 'harsh' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(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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