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