Loud, for one, seen in unique singer's exercise (7)

'singer's exercise' is the definition.
(exercise for learning musical notes)

'loud for one seen in unique' is the wordplay.
'loud' becomes 'f' (A musical term).
'for one' becomes 'eg' (I've seen this in other clues).
'seen in' means one lot of letters goes inside another.
'unique' becomes 'sole' ('sole' can be a synonym of 'unique').
'f'+'eg'='feg'
'feg' inserted inside 'sole' is 'SOLFEGE'.