Decree agreement has no power (3)
I believe the answer is:
act
'decree' is the definition.
(thesaurus)
'agreement has no power' is the wordplay.
'agreement' becomes 'pact' (pact is a kind of agreement).
'has no' suggests deleting specific letters.
'power' becomes 'p'.
'pact' with 'p' taken away is 'ACT'.
(Other definitions for act that I've seen before include "Major division of play" , "Routine" , "Take part in a play" , "Written piece of legislation" , "Perform(ance)" .)