Ellington? Not his first instrument! (3)
I believe the answer is:
uke
'instrument' is the definition.
(uke is a kind of instrument)
'ellington? not his first' is the wordplay.
'ellington?' becomes 'duke' (pianist).
'not his first' means to remove the first letter.
'duke' with its first letter taken off is 'UKE'.
(Other definitions for uke that I've seen before include "Notes producer sometimes plucked" , "Music producer" , "George Formby's instrument (abbr)" .)