Case: it involves singular person on the fiddle? (15)
I believe the answer is:
instrumentalist
'person on the fiddle?' is the definition.
(a violinist is a type of instrumentalist)
'case it' is the wordplay.
'case' becomes 'instrumental' (grammatical case in some languages).
'it' becomes 'ist' (I can't justify this - if you can you should believe this answer much more).
'instrumental'+'ist'='INSTRUMENTALIST'
'involves singular' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for instrumentalist that I've seen before include "oboe, perhaps" , "one of the band" , "musician" .)