Part of a fleet, maybe, leaves at sea with sun covering area (6)
I believe the answer is:
vessel
'part' is the definition.
(vessel is a kind of part)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for vessel that I've seen before include "Boat or bottle, say" , "Bark, possibly, needing water" , "Ship or large boat" , "Craft for water transportation" , "Container - ship" .)