Anti-aircraft fire (3-3)

I believe the answer is:

ack-ack

'anti-aircraft fire' is the definition.
(I've seen this before)

This is the entire clue.

(Other definitions for ack-ack that I've seen before include "Anti-aircraft gun" , "Anti-aircraft fire" , "Anti-aircraft gunfire" .)

I've seen this clue in The Independent, the Evening Standard and The Mirror.

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