The fat, soft piece (5)
I believe the answer is:
plump
'the fat' is the definition.
(I've seen this before)
'soft piece' is the wordplay.
'soft' becomes 'p' (musical abbreviation for piano, 'soft').
'piece' becomes 'lump' (lump is a kind of piece).
'p'+'lump'='PLUMP'
(Other definitions for plump that I've seen before include "Slightly fat" , "Well-rounded" , "Full of figure" , "Portly" , "Rather fat" .)