Fat, soft piece (5)
I believe the answer is:
plump
'fat' is the definition.
('plump' can be similar in meaning to 'fat')
'soft piece' is the wordplay.
'soft' becomes 'p' (musical abbreviation for 'piano').
'piece' becomes 'lump' (lump is a kind of piece).
'p'+'lump'='PLUMP'
(Other definitions for plump that I've seen before include "Full of figure" , "Pleasantly chubby" , "Chubby, rounded" , "Opt - round" , "Fat - fatten" .)