Marx perhaps had to leave when initially attacked with a weapon (9)

'leave when initially attacked with a weapon' is the definition.
I can't judge whether this defines the answer.

'marx perhaps had' is the wordplay.
'marx' becomes 'harpo' (Harpo Marx).
'perhaps' becomes 'one' (I can't explain this - if you can you should give a lot more credence to this answer).
'had' becomes 'd' (in contractions e.g. 'I'd', 'you'd' etc).
'harpo'+'one'+'d'='HARPOONED'

'to' is the link.

Can you help me to learn more?