First vessel to be capsized (3)
I believe the answer is:
top
'first' is the definition.
(synonyms)
'vessel to be capsized' is the wordplay.
'vessel' becomes 'pot' (type of container).
'to be capsized' shows that the letters should be reversed in order.
'pot' reversed gives 'TOP'.
(Other definitions for top that I've seen before include "Peak; child's toy" , "Highest part" , "Best" , "jumper, say?" , "Summit or spinning toy" .)