Period spent in strike? (5)

I believe the answer is:

knock

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'strike?' is the definition.
(knocking is a kind of striking)

I don't understand the rest of the clue.

(Other definitions for knock that I've seen before include "Sharp blow, perhaps on door" , "batsman's effort" , "Tap on door" , "Strike with a sharp blow" , "Thumping in engine" .)

I've seen this clue in The Times.

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