In the best shape for trial over border in front of magistrate (8)

I believe the answer is:

trimmest

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'in the best shape' is the definition.
I can't tell whether this defines the answer.

'trial over border in front of magistrate' is the wordplay.
'trial' becomes 'test' ('test' can be a synonym of 'trial').
'over' indicates putting letters inside.
'border in' becomes 'rim' (I've seen this before. I am not sure about the 'in' bit.**).
'front of' suggests taking the first letters.
The first letter of 'magistrate' is 'm'.
'rim'+'m'='rimm'
'test' placed around 'rimm' is 'TRIMMEST'.

'for' is the link.

Can you help me to learn more?

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

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