Leading vessel going backwards (3)
I believe the answer is:
top
'leading' is the definition.
(both can mean first-class)
'vessel going backwards' is the wordplay.
'vessel' becomes 'pot' (type of container).
'going backwards' is a reversal indicator.
'pot' back-to-front is 'TOP'.
(Other definitions for top that I've seen before include "Surface" , "Toy; uppermost point" , "Best" , "Acme or apex" , "Better" .)