Cunning deception disturbing in gem dealer (11)
I believe the answer is:
legerdemain
'cunning deception' is the definition.
(legerdemain can mean trickery)
'disturbing in gem dealer' is the wordplay.
'disturbing' indicates anagramming the letters (I've seen this in other clues).
'in'+'gem'+'dealer'='ingemdealer'
'ingemdealer' anagrammed gives 'LEGERDEMAIN'.
(Other definitions for legerdemain that I've seen before include "Find the lady will require this skill" , "Sleight of hand" , "Skill with hand" , "Magician's skill" , "leads to deception" .)