Instrument for singer with nothing on (7)
I believe the answer is:
bassoon
'instrument' is the definition.
(bassoon is a kind of instrument)
'singer with nothing on' is the wordplay.
'singer' becomes 'bass' (bass is a kind of singer).
'with' says to put letters next to each other.
'nothing' becomes 'o' (looks like zero - 0).
'bass'+'o'+'on'='BASSOON'
'for' acts as a link.
(Other definitions for bassoon that I've seen before include "Hollow instrument" , "Low wind instrument" , "Instrument - no ASBOs (anag)" , "Double-reed instrument" , "Instrument heard by Wedding-Guest" .)