Marines set lesson (7)
I believe the answer is:
seminar
'lesson' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'marines set' is the wordplay.
'set' indicates anagramming the letters (instruction to set the letters).
'marines' is an anagram of 'SEMINAR'.
Can you help me to learn more?
(Other definitions for seminar that I've seen before include "Tutorial group meeting" , "Group discussing" , "Student discussion group - remains (anag)" , "Marines (anag.)" , "Meeting for study" .)