St Mary staff dropping behind to knock off work (9)

'st mary' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'staff dropping behind to knock off work' is the wordplay.
'staff' becomes 'mace' (a mace is a ceremonial staff or stick).
'dropping behind' means to remove the last letter (the word's back or behind is dropped).
'to knock off' becomes 'kill' (knock off can mean to murder).
'work' becomes 'op' (abbreviation for opus).
'mace' with its last letter taken off is 'mac'.
'mac'+'kill'+'op'='MACKILLOP'

Can you help me to learn more?