It seems the vessel had cast off (4)
I believe the answer is:
shed
'cast off' is the definition.
(synonyms)
'it seems the vessel had' is the wordplay.
I cannot really understand how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found within the answer.
'had' could be 'd' (in contractions e.g. 'I'd', 'you'd' etc) and 'd' is present in the answer.
The remaining letters 'sh' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for shed that I've seen before include "Shack" , "shelter" , "Garden feature" , "Abandoned" , "that may be at bottom of garden?" .)