Quantity of beer popular with doctor, say? It sets the tone (6,3)
I believe the answer is:
tuning peg
'quantity of beer' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'popular with doctor say? it sets the tone' is the wordplay.
I cannot quite understand how this works, but
'popular' could be 'in' ('in' can mean trendy) and 'in' is located in the answer.
'doctor' could be 'gp' (general practitioner) and 'gp' is found in the answer.
'say?' could be 'eg' (both can mean for example) and 'eg' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
The remaining letters 'un' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for tuning peg that I've seen is " affects tightness of gut?".)