In between two notes, it's excellent (4)
I believe the answer is:
fine
'excellent' is the definition.
(synonyms)
'in between two notes' is the wordplay.
'two notes' can mean two substitutions for 'note'.
'between' is an insertion indicator.
'note' becomes 'f' (musical note).
'note' becomes 'e' (musical note).
'f'+'e'='fe'
'in' inserted inside 'fe' is 'FINE'.
'it's' acts as a link.
(Other definitions for fine that I've seen before include "Super" , "musical ending" , "nearly in line with wicket" , "Very high quality money penalty" , "Fee payable for misdemeanour" .)