Musical hard on a poet (8)
I believe the answer is:
hamilton
'musical' is the definition.
'hard on a poet' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'on' is a charade indicator (letters next to each other).
'poet' becomes 'milton' (John Milton).
'h'+'a'+'milton'='HAMILTON'
(Other definitions for hamilton that I've seen before include "city down under" , "Lady . . . . . . . . was Lord Nelson's lover" , "Scottish town" , "Emma -, Nelson's mistress" , "Lady mistress of Napoleon" .)