Month amphibious troops originally introduced Dutch soldiers (9)
I believe the answer is:
thermidor
'month' is the definition.
(Thermidor is an example)
'amphibious troops originally introduced dutch soldiers' is the wordplay.
'amphibious troops' becomes 'the rm' (the Royal Marines).
'originally' indicates taking the first letters.
'soldiers' becomes 'OR' (Other Ranks).
The first letters of 'introduced dutch' is 'id'.
'therm'+'id'+'or'='THERMIDOR'
(Other definitions for thermidor that I've seen before include "month of revolution" , "month in Paris once?" , "Revolutionary month (of the lobster?)" , "Method of cooking lobster" .)