Agreement sounds full (4)
I believe the answer is:
pact
'agreement' is the definition.
(a pact is an agreement or deal)
'sounds full' is the wordplay.
'sounds' indicates a 'sounds like' (homophone) clue.
'full' becomes 'packed' (synonyms).
'packed' sounds like 'PACT'.
(Other definitions for pact that I've seen before include "International treaty" , "Formal agreement between parties or states" , "Treaty, formal agreement" , "deal" , "Agreement, covenant" .)