The Royal Marines fish (gold) one month in France (9)
I believe the answer is:
thermidor
'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" .)