It's a big fiddle (5)
I believe the answer is:
viola
'fiddle' is the definition.
(I've seen this before)
'it's a big' is the wordplay.
I cannot quite see how this works, but
'a' is found in the answer.
The remaining letters 'viol' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for viola that I've seen before include "Musical instrument "Twelfth Night" character" , "Bowed string instrument" , "Large violin" , "Page" , "Shakespearean character" .)