Small, soft lament (4)
I believe the answer is:
weep
'lament' is the definition.
(both can mean to cry)
'small soft' is the wordplay.
'small' becomes 'wee' (Scottish term for small).
'soft' becomes 'p' (musical abbreviation for 'piano').
'wee'+'p'='WEEP'
(Other definitions for weep that I've seen before include "Pee-pee" , "Cry, shed tears" , "Keen" , "Greet" , "pipe" .)