Musical instrument a singer gets on with (7)
I believe the answer is:
bassoon
'musical instrument' is the definition.
(bassoon is a kind of musical instrument)
'a singer gets on with' is the wordplay.
'a singer' becomes 'bass' (bass is a kind of singer).
'gets on with' becomes 'oon' (I can't explain this - if you can you should give a lot more credence to this answer).
'bass'+'oon'='BASSOON'
Can you help me to learn more?
(Other definitions for bassoon that I've seen before include "Low wind instrument" , "Instrument - no ASBOs (anag)" , "Double-reed instrument" , "No, a boss would be a musical instrument" , "So-so ban on the instruments" .)