Dodgy dealer purchases minute gemstone (7)
I believe the answer is:
emerald
'gemstone' is the definition.
(emerald is a kind of gemstone)
'dodgy dealer purchases minute' is the wordplay.
'dodgy' indicates an anagram.
'purchases' means one lot of letters goes inside another.
'minute' becomes 'm' (abbreviation).
'dealer' with letters rearranged gives 'eerald'.
'eerald' going around 'm' is 'EMERALD'.
(Other definitions for emerald that I've seen before include "Green gem" , "and 1 Across: Ireland" , "Bright green colour" , "Highly-valued variety of beryl" , "Gem; type of moth" .)