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