Write soft letters (3)
I believe the answer is:
pen
'write' is the definition.
(synonyms)
'soft letters' is the wordplay.
'soft' becomes 'p' (musical abbreviation for 'piano').
'letters' becomes 'en' (I can't justify this - if you can you should give a lot more credence to this answer).
'p'+'en'='PEN'
Can you help me to learn more?
(Other definitions for pen that I've seen before include "Confine the writer" , "Female swan" , "dash off" , "American jug?" , "Swan for the writer" .)