Crooked dealer pocketing millions for gemstone (7)
I believe the answer is:
emerald
'for gemstone' is the definition.
(emerald is a kind of gemstone)
'crooked dealer pocketing millions' is the wordplay.
'crooked' indicates anagramming the letters (a crooked or disrupted version of the letters).
'pocketing' is an insertion indicator (pocket can mean to steal or take possession of).
'millions' becomes 'm' (abbreviation).
'dealer' anagrammed gives 'eerald'.
'eerald' going around 'm' is 'EMERALD'.
(Other definitions for emerald that I've seen before include "La couleur" , "Bright green colour" , "Green like this isle" , "Green gemstone" , "Precious gemstone" .)