He might ruin Pat, possibly (7)

I believe the answer is:

puritan

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'he might' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

'ruin pat possibly' is the wordplay.
'possibly' indicates an anagram.
'ruin'+'pat'='ruinpat'
'ruinpat' anagrammed gives 'PURITAN'.

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(Other definitions for puritan that I've seen before include "Extreme moralist" , "Strait-laced" , "Person of extreme moral strictness" , "One with very strict moral principles" , "Killjoy" .)

I've seen this clue in The Irish Independent.

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