Musical notes, allegro, mostly? (3)
I believe the answer is:
fas
'musical notes' is the definition.
(more than one of the musical note 'fa')
'allegro mostly?' is the wordplay.
'allegro' becomes 'fast' (similar in meaning).
'mostly?' means to remove the last letter (I've seen 'mostly' mean this).
'fast' with its last letter taken off is 'FAS'.
I've seen this clue in The Times.
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