Rotter that retains cat's haul? (6)

I believe the answer is:

ratbag

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'cat's haul?' is the definition.
The definition suggests a singular noun which matches the answer.

'rotter that retains' is the wordplay.
'rotter' becomes 'rat' (synonyms).
'that retains' becomes 'bag' (I can't explain this - if you can you should give a lot more credence to this answer).
'rat'+'bag'='RATBAG'

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(Other definitions for ratbag that I've seen before include "unpleasant customer" , "Grab at (anag) - contemptible person" , "One displaying bad form" , "snake" , "bounder" .)

I've seen this clue in The Guardian.

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