Peculiar piano seat (4)
I believe the answer is:
rump
'seat' is the definition.
(I know that seat can be written as rump)
'peculiar piano' is the wordplay.
'peculiar' becomes 'rum' (synonyms).
'piano' becomes 'p' (musical abbreviation).
'rum'+'p'='RUMP'
(Other definitions for rump that I've seen before include "Hind part; small remnant" , "Rear; buttocks" , "Cut of meat from the rear" , "Hind part of body" , "Mammal's hind part" .)