Soon some new piano number (4)
I believe the answer is:
anon
'soon' is the definition.
(synonyms)
'some new piano number' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'ANON' can be found hidden inside 'new piano number'.
(Other definitions for anon that I've seen before include "Soon, precious stone" , "In due course" , "Without known author" , "Nameless (abbr.); soon" , "Soon, shortly (archaic)" .)