Commandos transported by vessel, obnoxious types (6)
I believe the answer is:
vermin
'obnoxious types' is the definition.
(vermin can mean obnoxious people)
'commandos transported by vessel' is the wordplay.
'commandos' becomes 'RM' (Royal Marines).
'transported by' indicates putting letters inside (some letters carried or transported by others).
'vessel' becomes 'vein' (vein is a kind of vessel).
'rm' going inside 'vein' is 'VERMIN'.
(Other definitions for vermin that I've seen before include "Obnoxious people" , "Harmful or disease-ridden animals" , "eg Mice" , "Annoying small animals or insects" , "Pestilent creatures" .)