Very hard part of scale (3)
I believe the answer is:
soh
'part of scale' is the definition.
'soh' can be an answer for 'part' (soh is a kind of part). I'm not sure about the 'of scale' bit.
'very hard' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'hard' becomes 'h' (abbreviation used in pencil classifications).
'so'+'h'='SOH'
Can you help me to learn more?
(Other definitions for soh that I've seen before include "element of scale" , "It comes between fah and lah" , "Note of major scale" , "It's between fah and la" , "Doh ray me fah . . ." .)