First pair of marines aboard harbour vessel, back in range (5)
I believe the answer is:
gamut
'range' is the definition.
(gamut can mean a range or variety)
'first pair of marines aboard harbour vessel back' is the wordplay.
'first' becomes 'a'.
'pair of marines' becomes 'm' (I can't justify this - if you can you should believe this answer much more).
'aboard' indicates putting letters inside.
'harbour vessel' becomes 'tug' (tugboat).
'back' is a reversal indicator.
'tug' in reverse letter order is 'gut'.
'a'+'m'='am'
'am' put within 'gut' is 'GAMUT'.
'in' is the link.
(Other definitions for gamut that I've seen before include "Full (emotional) range" , "The whole range of, say emotions" , "Whole range (from A to Z)" , "Complete range (of something)" , "The whole range or scope of anything" .)