Musical note held by fine old mum (8)
I believe the answer is:
oklahoma
'musical' is the definition.
(I've seen this before)
'note held by fine old mum' is the wordplay.
'note' becomes 'lah' (I've seen this before**).
'held by' means one lot of letters goes inside another.
'fine' becomes 'ok' ('OK' can be a synonym of 'fine').
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'mum' becomes 'ma' ('ma' can be a synonym of 'mum').
'ok'+'o'='oko'
'lah' inserted into 'oko' is 'oklaho'.
'oklaho'+'ma'='OKLAHOMA'
(Other definitions for oklahoma that I've seen before include "A holm oak found in the state" , "Look, a ham from the state" , "state of America" , "A holm oak found over in the States" , "American state and great western musical" .)