Number boarding vessel, strong (6)
I believe the answer is:
potent
'strong' is the definition.
('potent' can be a synonym of 'strong')
'number boarding vessel' is the wordplay.
'number' becomes 'ten' (ten is a kind of number).
'boarding' is an insertion indicator.
'vessel' becomes 'pot' (pot is a kind of vessel**).
'ten' put inside 'pot' is 'POTENT'.
(Other definitions for potent that I've seen before include "Powerful or effective" , "Powerful like strong drink" , "Strong or concentrated, like some drink" , "Forceful" , "Strong or effective" .)