Diamonds and heroin on vessel: an unknown quantity (4,5)
I believe the answer is:
dark horse
'an unknown quantity' is the definition.
(I've seen this before)
'diamonds and heroin on vessel' is the wordplay.
'diamonds' becomes 'd' (abbreviation in bridge say).
'and' means one lot of letters go next to another.
'heroin' becomes 'horse' (slang term for heroin).
'on' is a charade indicator (letters next to each other).
'vessel' becomes 'ark' (ark is a kind of vessel).
'horse' put after 'ark' is 'arkhorse'.
'd'+'arkhorse' is 'DARK HORSE'.
(Other definitions for dark horse that I've seen before include "Competitor whose abilities are unknown" , "Little is known of its properties" , "Little-known competitor like shadowy equine" , "Unexpected (possible) winner" , "Successful competitor of whom little was known, like colourless equine" .)