Gem dealer abroad banking millions (7)
I believe the answer is:
emerald
'gem' is the definition.
(emerald is a kind of gem)
'dealer abroad banking millions' is the wordplay.
'abroad' indicates an anagram.
'banking' is an insertion indicator.
'millions' becomes 'm' (abbreviation).
'dealer' is an anagram of 'eerald'.
'eerald' enclosing 'm' is 'EMERALD'.
(Other definitions for emerald that I've seen before include "Led mare (anag.)" , "Precious green stone" , "Bright green colour" , "Gem associated with Ireland" , "On of the Greens" .)