Heard agreement is full (6)
I believe the answer is:
packed
'is full' is the definition.
(synonyms)
'heard agreement' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'agreement' becomes 'pact' (a pact is an agreement or deal).
'pact' sounds like 'PACKED'.
(Other definitions for packed that I've seen before include "Enveloped" , "Placed clothes etc in suitcase" , "Arranged clothes in suitcase for travelling" , "Crowded" .)