Formal agreement dropping the first parliamentary statute (3)
I believe the answer is:
act
'statute' is the definition.
(thesaurus)
'formal agreement dropping the first parliamentary' is the wordplay.
'formal agreement' becomes 'pact' (I've seen this before).
'dropping' suggests deleting specific letters.
'the first parliamentary' becomes 'p' (1st letter of 'parliamentary').
'pact' with 'p' taken away is 'ACT'.
(Other definitions for act that I've seen before include "A deed or statute" , "Written piece of legislation" , "Perform - part of a play" , "Perform theatrically" , "Function" .)