Crossword Genius

The Royal Marines fish (gold) one month in France (9)

Ross

I believe the answer is:

thermidor

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

'france' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

'the royal marines fish gold one month in' is the wordplay.
'royal marines fish' becomes 'rid' (I can't justify this - if you can you should give a lot more credence to this answer).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'one month' becomes 'M' (standard abbreviation).
'in' indicates putting letters inside.
'rid'+'or'='ridor'
'ridor' placed around 'm' is 'rmidor'.
'the'+'rmidor'='THERMIDOR'

Can you help me to learn more?

(Other definitions for thermidor that I've seen before include "month in Paris once?" , "Revolutionary month (of the lobster?)" , "Method of cooking lobster" , "month of revolution" .)

I've seen this clue in the Sydney Morning Herald.
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