Crossword Genius

What might ruin mother in a tribunal battle (9)

Ross

I believe the answer is:

agincourt

I'm a little stuck... Click here to teach me more about this clue!

'battle' is the definition.
(Agincourt is an example)

'what might ruin mother in a tribunal' is the wordplay.
I cannot quite understand how this works, but
'in' is found in the answer.
'a' is present in the answer.
'tribunal' could be 'court' (synonyms) and 'court' is found in the answer.
A single letter 'g' remains which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for agincourt that I've seen before include "Battleground" , "Henry V's battle" , "Battle in Hundred Years' War" , "French village" , "where Henry met Constable" .)

I've seen this clue in The Sunday Times.
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