Switch to fiddle used by old composer (7)
I believe the answer is:
rodrigo
'composer' is the definition.
(I've seen this before)
'switch to fiddle used by old' is the wordplay.
I cannot quite see how this works, but
'switch' could be 'rod' (I've seen this in another clue) and 'rod' is found within the answer.
'fiddle' could be 'rig' (I've seen this before) and 'rig' is located in the answer.
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rodrigo that I've seen before include "Resident caretaker (in France)" , "Spanish composer (Concierto de Aranjuez)" , "Spanish scorer" , "Joaqoein -, Concierto de Aranjuez composer" .)