Dickensian crook heading off to case area once more (5)
I believe the answer is:
again
'once more' is the definition.
(synonyms)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for again that I've seen before include "Not for the first time" , "One more time, encore" , "Predictably perhaps" , "Anew" , "Repeatedly" .)