Agreement with no power in law (3)
I believe the answer is:
act
'law' is the definition.
(thesaurus)
'agreement with no power' is the wordplay.
'agreement' becomes 'pact' (pact is a kind of agreement).
'with no' indicates named letters should be taken away.
'power' becomes 'p'.
'pact' with 'p' taken out is 'ACT'.
'in' is the link.
(Other definitions for act that I've seen before include "Subdivision of a play" , "Accomplishment" , "Thing done" , "Pretend to be - deed" , "Written piece of legislation" .)