"in the main" (6)
'in the main' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for largely that I've seen before include "Generally" , "Mainly" , "To a great extent" , "to a considerable extent" , "Fro the most part" .)
(Other definitions for marine that I've seen before include "Oceanic" , "Maritime - seagoing soldier" , "Soldier associated with a navy" , "Soldier-sailor" , "Member of the forces" .)