Soft, ultimately sweet darling (3)
I believe the answer is:
pet
'darling' is the definition.
(both are terms of endearment)
'soft ultimately sweet' is the wordplay.
'soft' becomes 'p' (musical abbreviation for 'piano').
'ultimately sweet' becomes 'et' (I can't justify this - if you can you should believe this answer much more).
'p'+'et'='PET'
Can you help me to learn more?
(Other definitions for pet that I've seen before include "Preferred" , "Cat or dog" , "House animal; sulk" , "The darling one in a small temper" , "Loved domestic animal" .)