What sounds like full agreement (4)
I believe the answer is:
pact
'agreement' is the definition.
(a pact is an agreement or deal)
'what sounds like full' is the wordplay.
'what sounds like' shows a homophone (sound like).
'full' becomes 'packed' (synonyms).
'packed' is a homophone of 'PACT'.
(Other definitions for pact that I've seen before include "Agreement, bargain" , "Settlement" , "'Accord, treaty (4)'" , "Agreement, covenant" , "International treaty" .)