A warning shout before the plane crashes to the ground? (6)

I believe the answer is:

timber

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'a warning' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for timber that I've seen before include "22 [BEAM]" , "board" , "Feller's warning cry" , "Ash perhaps" , "Watch out for falling tree!" .)

I've seen this clue in the Evening Standard.

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