Men in a docked brown vessel (5)
I believe the answer is:
aorta
'vessel' is the definition.
(aorta is a kind of vessel)
'men in a docked brown' is the wordplay.
'men' becomes 'or' (I've seen this in other clues).
'in' is an insertion indicator.
'docked' means to remove the last letter (to dock can mean to remove the tail of an animal).
'brown' becomes 'tan' (tan is a kind of brown).
'tan' with its last letter taken away is 'ta'.
'a'+'ta'='ata'
'or' placed within 'ata' is 'AORTA'.
(Other definitions for aorta that I've seen before include "Body's main artery" , "Heart's great artery" , "Major blood vessel" , "blood flows from it" , "claret supplier" .)