Old vessel, unlit, heading off (3)
I believe the answer is:
ark
'old vessel' is the definition.
(I've seen this before)
'unlit heading off' is the wordplay.
'unlit' becomes 'dark' (similar in meaning).
'heading off' means to remove the first letter.
'dark' with its initial letter taken off is 'ARK'.
(Other definitions for ark that I've seen before include "Coffer" , "old vessel" , "Rescue boat" , "protection for couples that rose in bad weather" , "Great biblical boat" .)