Proves company is involved in fraudulent activities (8)
I believe the answer is:
confirms
'proves' is the definition.
(to prove is to confirm as true)
'company is involved in fraudulent activities' is the wordplay.
'company' becomes 'firm' (firm can mean a company or business).
'is involved in' means one lot of letters goes inside another.
'fraudulent activities' becomes 'cons' (a con is a fraudulent operation).
'firm' going into 'cons' is 'CONFIRMS'.
(Other definitions for confirms that I've seen before include "Finalises or makes definite" , "Provides support for the truth of" , "Provides supporting evidence of the truth" , "Supports the truth of something" , "Corroborates" .)