Marines board vessel to find pests (6)
I believe the answer is:
vermin
'pests' is the definition.
(I've seen this before)
'marines board vessel' is the wordplay.
'marines' becomes 'rm' (abbreviation for 'Royal Marines').
'board' means one lot of letters goes inside another.
'vessel' becomes 'vein' (vein is a kind of vessel**).
'rm' going within 'vein' is 'VERMIN'.
'to find' is the link.
(Other definitions for vermin that I've seen before include "Common animals, birds or parasitic insects" , "Pests, rats say" , "Rodents that spread disease" , "Pestilent creatures" , "Unwanted creature(s)" .)