Behaved oneself when one stood in for (5)

I believe the answer is:

acted

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'behaved' is the definition.
(acting is a kind of behaving)

I can't explain the remainder of the clue.

(Other definitions for acted that I've seen before include "Behaved in a certain manner" , "Played a 30ac" , "took part in simulation" , "Took part in play" , "Performed (in a play)" .)

I've seen this clue in the Evening Standard.

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