Note men on Territorial Army vessel (5)
I believe the answer is:
aorta
'vessel' is the definition.
(aorta is a kind of vessel)
'note men on territorial army' is the wordplay.
'note' becomes 'a' (musical note).
'men' becomes 'or' (I've seen this in other clues).
'on' means one lot of letters go next to another.
'territorial army' becomes 'ta'.
'a'+'or'+'ta'='AORTA'
(Other definitions for aorta that I've seen before include "vital transporter" , "Blood-carrier from a heart" , "Heart's great artery" , "Blood carrier" , "claret supplier" .)