Person in court is restrained over argument (9)

I believe the answer is:

plaintiff

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'person in court' is the definition.

'restrained over argument' is the wordplay.
'restrained over' becomes 'plain' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'argument' becomes 'tiff' (I've seen this before).
'plain'+'tiff'='PLAINTIFF'

'is' is the link.

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(Other definitions for plaintiff that I've seen before include "One bringing civil court case" , "One brings a legal case against another" , "Claimant" , "Person initiating lawsuit" , "Person who brings a legal action" .)

I've seen this clue in The Times.

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