Marines off to a conference (7)
I believe the answer is:
seminar
'to a conference' is the definition.
(seminar is a kind of conference)
'marines off' is the wordplay.
'off' is an anagram indicator.
'marines' anagrammed gives 'SEMINAR'.
(Other definitions for seminar that I've seen before include "Training conference" , "Group discussion of subject" , "Study group on specific subject" , "Marines (anag.)" , "Meeting for study" .)