Classes of marine trained on board ship (8)
I believe the answer is:
seminars
'classes' is the definition.
(seminar is a kind of class)
'marine trained on board ship' is the wordplay.
'trained' indicates an anagram (to train can mean to grow something into a shape).
'on board' indicates putting letters inside.
'ship' becomes 'ss' (**).
'marine' is an anagram of 'eminar'.
'eminar' going into 'ss' is 'SEMINARS'.
'of' acts as a link.
(Other definitions for seminars that I've seen before include "workshops" , "classes" , "discussions are academic" .)