How two auxiliary verbs act (6)

I believe the answer is:

behave

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'act' is the definition.
(I know that act can be written as behave)

I don't understand the remainder of the clue.

(Other definitions for behave that I've seen before include "Act in a certain manner" , "Conduct oneself properly" , "Act or react - show good manners" , "Comport oneself" , "Act appropriately" .)

I've seen this clue in The Globe and Mail.

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