Additional individual abetting criminal (9)
I believe the answer is:
accessory
'additional' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for accessory that I've seen before include "Useful added item" , "Someone who helps another commit a crime" , "Supplementary item" , "Item such as a handbag that goes with a dress" , "Criminal's helper" .)