Workshop marines destroyed (7)
I believe the answer is:
seminar
'workshop' is the definition.
(I've seen this before)
'marines destroyed' is the wordplay.
'destroyed' indicates an anagram.
'marines' is an anagram of 'SEMINAR'.
(Other definitions for seminar that I've seen before include "Training conference" , "Teaching meeting, conference" , "Discussion meeting" , "University course" , "Meeting for exchange of ideas" .)