Acts up without us in agreement (4)

I believe the answer is:

pact

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'agreement' is the definition.
(pact is a kind of agreement)

I don't understand the remainder of the clue.

(Other definitions for pact that I've seen before include "Written agreement between two states" , "Treaty, formal agreement" , "'Accord, treaty (4)'" , "Agreement, bargain" , "Alliance" .)

I've seen this clue in the Irish Times.

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