Gemstone dealer Mark Mills (7)
I believe the answer is:
emerald
'gemstone' is the definition.
(emerald is a kind of gemstone)
'dealer mark mills' is the wordplay.
I cannot quite understand how this works, but
'mark' could be 'm' and 'm' is found in the answer.
an anagram of 'dealer' is 'eerald' which is within the left over letters.
No letters remain.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for emerald that I've seen before include "La couleur" , "Highly-valued variety of beryl" , "a type employed by the press" , "Led mare (anag.)" , "Green - jewel" .)