Extent of some musical exercise (5)
I believe the answer is:
scale
I believe this is a double definition.
'extent' is the first definition.
(I've seen this before)
'some musical exercise' is the second definition.
(I've seen this in another clue)
'of' is the link.
(Other definitions for scale that I've seen before include "Need balance" , "Doh, ray, me" , "Alec's (anag) -- graduation" , "An ordered series of units for measurement purposes" , "plaque" .)