Crossword Genius

Assists investigator finally in exposing illicit exchange of favours (14)

Ross

I believe the answer is:

backscratching

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Hindu.
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