Unknown quantity of diamonds and heroin ship carried (4,5)
I believe the answer is:
dark horse
'unknown quantity' is the definition.
(I've seen this before)
'diamonds and heroin ship carried' is the wordplay.
'diamonds' becomes 'd' (abbreviation for the card suit diamonds).
'and' says to put letters next to each other.
'heroin' becomes 'horse' (slang term for heroin).
'ship' becomes 'ark' (I've seen this in other clues).
'carried' means one lot of letters goes inside another (one word carried by another word).
'd'+'horse'='dhorse'
'dhorse' placed around 'ark' is 'DARK HORSE'.
'of' is the link.
(Other definitions for dark horse that I've seen before include "Unknown quantity" , "Successful competitor of whom little was known, like colourless equine" , "Competitor whose abilities are unknown" , "Unexpected (possible) winner" , "Little is known of its properties" .)