Assists investigator finally in exposing illicit exchange of favours (14)
I believe the answer is:
backscratching
'illicit exchange of favours' is the definition.
I can't tell whether this defines the answer.
'assists investigator finally in' is the wordplay.
I cannot really see how this works, but
'in' is within the answer.
This may be the basis of clue (or it may be nonsense).
'exposing' acts as a link.
Can you help me to learn more?
(Other definitions for backscratching that I've seen before include "performing mutual favours" , "could be a relief" .)