Point proved mercy-seat is mysterious (1,4,2,4)

I believe the answer is:

i rest my case

'point proved' is the definition.

'mercy-seat is mysterious' is the wordplay.
'mysterious' is an anagram indicator.
'mercy'+'seat'+'is'='mercyseatis'
'mercyseatis' is an anagram of 'I REST MY CASE'.

(Another definition for i rest my case that I've seen is " That's all I need to say".)

I've seen this clue in The Telegraph.

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