Crooked dealer taking in a thousand for the gem (7)
I believe the answer is:
emerald
'the gem' is the definition.
(type of gem stone)
'crooked dealer taking in a thousand' is the wordplay.
'crooked' indicates anagramming the letters (a crooked or disrupted version of the letters).
'taking in' indicates putting letters inside.
'a thousand' becomes 'M' (Roman numeral for a thousand).
'dealer' with letters rearranged gives 'eerald'.
'eerald' going around 'm' is 'EMERALD'.
'for' acts as a link.
(Other definitions for emerald that I've seen before include "Led mare (anag.)" , "La couleur" , "city in Oz?" , "and 13 Across: Poetic name for Ireland" , "a type employed by the press" .)