Treaty wrapped up in hearing (4)
I believe the answer is:
pact
'treaty' is the definition.
(I know that pact is a type of treaty)
'wrapped up in hearing' is the wordplay.
'wrapped up' becomes 'packed'.
'in hearing' indicates a 'sounds like' (homophone) clue.
'packed' sounds like 'PACT'.
(Other definitions for pact that I've seen before include "Settlement" , "'Accord, treaty (4)'" , "Treaty, formal agreement" , "Agreement, bargain" , "Agreement, covenant" .)