Thief's run inside as client worked (9)
I believe the answer is:
Here is my best explanation:
'thief's' is the definition.
(larcenist is a kind of thief)
'run inside as client worked' is the subsidiary indication.
'run' becomes 'r' (cricket abbreviation).
'inside' means one lot of letters goes inside another.
'worked' is an anagram indicator.
'asclient' is an anagram of 'lacenist'.
'r' put inside 'lacenist' is 'larcenist'.
(Other definitions for larcenist that I've seen before include "Person who takes", "Thief", "Criminal", "Dishonest person", "One criminal", "One takes", "nicker" and "One steals clarinets, oddly".)