Law introduced with leader of parliament’s agreement (4)

I believe the answer is:

pact

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'agreement' is the definition.
(a pact is an agreement or deal)

I can't explain the rest of the clue.

(Other definitions for pact that I've seen before include "Agreement, covenant" , "Settlement" , "deal" , "International treaty" , "Alliance" .)

I've seen this clue in The Times.

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