Leaving musical, son is being pedantic (9,5)
I believe the answer is:
splitting hairs
'being pedantic' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'leaving musical son' is the wordplay.
I cannot quite see how this works, but
'musical' could be 'hair' (1960s musical) and 'hair' is present in the answer.
'son' could be 's' (genealogical abbreviation for son) and 's' is located in the answer.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Another definition for splitting hairs that I've seen is " Making fine distinctions".)