Quietly play part in treaty (4)
I believe the answer is:
pact
'treaty' is the definition.
('pact' can be a synonym of 'treaty')
'quietly play part' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'play part' becomes 'act' ('act' is part of 'play'**).
'p'+'act'='PACT'
'in' is the link.
(Other definitions for pact that I've seen before include "Formal agreement between parties or states" , "Alliance" , "Settlement" , "'Accord, treaty (4)'" , "deal" .)