Best vessel brought up (3)
I believe the answer is:
top
'best' is the definition.
(top can mean the very best)
'vessel brought up' is the wordplay.
'vessel' becomes 'pot' (type of container).
'brought up' shows that the letters should be reversed in order (in a down clue, letters go up).
'pot' in reverse letter order is 'TOP'.
(Other definitions for top that I've seen before include "Highest point or position" , "Zenith" , "Acme or apex" , "The upper part" , "Highest, child's toy" .)