Cutting very critical leading actor to be replaced by unknown (8)

'cutting' is the definition.
The answer and definition can be both to do with contact as well as being present participle verbs.
Perhaps you can see an association between them that I can't see?

'critical leading actor to be replaced by unknown' is the wordplay.
I cannot quite understand how this works, but
'leading' could be 'in' (in government) and 'in' is found within the answer.
'unknown' could be 'y' (algebra) and 'y' is located in the answer.
This may be the basis of the clue (or it may be nonsense).

'very' is the link.

Can you help me to learn more?