Stolen drug found in repeated check in mixture (10)
I believe the answer is:
hotchpotch
'mixture' is the definition.
(hotchpotch is a kind of mixture)
'stolen drug found in repeated check' is the wordplay.
'stolen' becomes 'hot' (I've seen this before).
'drug' becomes 'pot' (slang term for cannabis).
'found in' is an insertion indicator.
'repeated' shows that letters should be duplicated.
'check' becomes 'ch' (I've seen this in other clues).
'ch' duplicated is 'chch'.
'pot' put inside 'chch' is 'chpotch'.
'hot'+'chpotch'='HOTCHPOTCH'
'in' acts as a link.
(Other definitions for hotchpotch that I've seen before include "Confused medley" , "jumble" , "Confused mixture" , "confusion" , "Mutton stew" .)