Tars at sea circling lake in vessels (8)
I believe the answer is:
arteries
'vessels' is the definition.
(artery is a kind of vessel)
'tars at sea circling lake' is the wordplay.
'at sea' indicates anagramming the letters.
'circling' is an insertion indicator (circling can mean going around or containing).
'lake' becomes 'erie' (Lake Erie is one of the Canadian/US 'Great Lakes').
'tars' anagrammed gives 'arts'.
'arts' going around 'erie' is 'ARTERIES'.
'in' is the link.
(Other definitions for arteries that I've seen before include "Vessels that carry blood from the heart to the body" , "Blood vessels leading from the heart" , "Major thoroughfares for important traffic" , "Blood vessels - major roads" .)