A number in the vessel are powerful (6)
I believe the answer is:
potent
'are powerful' is the definition.
(associated in meaning)
'a number in the vessel' is the wordplay.
'a number' becomes 'ten' (ten is a number).
'in the' means one lot of letters goes inside another.
'vessel' becomes 'pot' (type of container).
'ten' put within 'pot' is 'POTENT'.
(Other definitions for potent that I've seen before include "Powerful like strong drink" , "Having great power" , "Powerful or effective" , "Powerful, influential" , "Concentrated or strong, like liquor" .)