Stop playing piano? Sad (8)
I believe the answer is:
diapason
'stop' is the definition.
(thesaurus)
'playing piano? sad' is the wordplay.
'playing' is an anagram indicator.
'piano'+'sad'='pianosad'
'pianosad' is an anagram of 'DIAPASON'.
(Other definitions for diapason that I've seen before include "Organ stop; sad piano (anag.)" , "tuning fork" , "Othello's enemy" , "Entire range of musical instrument or voice" .)