Put on a soft layer (5)
I believe the answer is:
apply
'put on' is the definition.
(I've seen this before)
'a soft layer' is the wordplay.
'soft' becomes 'p' (musical abbreviation for 'piano').
'layer' becomes 'ply' (ply is a kind of layer).
'a'+'p'+'ply'='APPLY'
(Other definitions for apply that I've seen before include "describing the cooker?" , "Make a formal request (to someone); put on" , "Employ; be relevant" , "concentrate" , "Send off (form, etc)" .)