Telltale thief (5)
I believe the answer is:
sneak
I believe this is a double definition.
'telltale' is the first definition.
(I've seen this before)
'thief' is the second definition.
(sneak thief is a kind of thief)
(Other definitions for sneak that I've seen before include "thief" , "peach" , "To go stealthily" , "Go furtively - inform on another child" , "Prowl" .)