Soft stuff in buggy (4)
I believe the answer is:
pram
'buggy' is the definition.
(a buggy for children)
'soft stuff' is the wordplay.
'soft' becomes 'p' (musical abbreviation for 'piano').
'stuff' becomes 'ram' (ram can mean to stuff or push).
'p'+'ram'='PRAM'
'in' acts as a link.
(Other definitions for pram that I've seen before include "A common baby carriage" , "early transport" , "Ramp for a baby buggy" , "Nanny may use this" , "Cargo-boat" .)