For a cat to turn up with me, could spoil things (4,2)

I believe the answer is:

mess up

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'spoil things' is the definition.
'mess up' can be an answer for 'spoil' ('mess up' can be a synonym of 'spoil'). I'm not sure about the 'things' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for mess up that I've seen before include "Botch" , "Muddle - confusion" , "Fluff" , "Mishandle a situation" , "ruin" .)

I've seen this clue in the Evening Standard.

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