Agreement stuffed, by the sound of it? (4)
I believe the answer is:
pact
'agreement' is the definition.
(pact is a kind of agreement)
'stuffed by the sound of it?' is the wordplay.
'stuffed' becomes 'packed' (packing is a kind of stuffing).
'by the sound of it?' shows a homophone (sound like).
'packed' sounds like 'PACT'.
(Other definitions for pact that I've seen before include "Treaty, formal agreement" , "International treaty" , "Formal agreement between parties or states" , "Agreement, bargain" , "Agreement, covenant" .)