A sweet, ultimately soft drink (5)
I believe the answer is:
julep
'drink' is the definition.
(julep is a kind of drink)
'a sweet ultimately soft' is the wordplay.
'a sweet ultimately' becomes 'jule' (I can't justify this - if you can you should believe this answer much more).
'soft' becomes 'p' (musical abbreviation for 'piano').
'jule'+'p'='JULEP'
Can you help me to learn more?
(Other definitions for julep that I've seen before include "Cocktail of bourbon, sugar and mint over crushed ice" , "(Minted) drink made with bourbon, sugar and crushed ice" , "Sweet drink, often medicated" , "See 21 Down" , "Mint drink in America" .)