Leaving influence? That's sick (9)

I believe the answer is:

offcolour

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'sick' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'leaving influence?' is the wordplay.
'leaving' becomes 'off' (as in 'I'm off').
'influence?' becomes 'colour' (I've seen this before).
'off'+'colour'='OFFCOLOUR'

'that's' is the link.

Can you help me to learn more?

I've seen this clue in The Daily Express.

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