Main line (5)
'main line' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aorta that I've seen before include "One leads from heart" , "main arterial route" , "Trunk artery from the heart" , "Blood-carrier from a heart" , "Major artery at left of heart" .)
(Other definitions for coast that I've seen before include "Shoreline" , "Sea border" , "Diminish, lessen" , "Where the land and sea meet" , "Move easily without power" .)