Treaty sounds full (4)
I believe the answer is:
pact
'treaty' is the definition.
(I know that treaty can be written as pact)
'sounds full' is the wordplay.
'sounds' indicates a 'sounds like' (homophone) clue.
'full' becomes 'packed' (synonyms).
'packed' is a homophone of 'PACT'.
(Other definitions for pact that I've seen before include "Formal agreement between parties or states" , "Settlement" , "deal" , "Written agreement between two states" , "Agreement, bargain" .)