Agreement sound full (4)
I believe the answer is:
pact
'agreement' is the definition.
(pact is a kind of agreement)
'sound full' is the wordplay.
'sound' shows a homophone (sound like).
'full' becomes 'packed' ('packed' can be a synonym of 'full').
'packed' is a homophone of 'PACT'.
(Other definitions for pact that I've seen before include "'Accord, treaty (4)'" , "Agreement, bargain" , "Settlement" , "deal" , "Written agreement between two states" .)