Virus containing note getting illicit payment (4)
I believe the answer is:
bung
'illicit payment' is the definition.
'virus containing note' is the wordplay.
'virus' becomes 'bug'.
'containing' means one lot of letters goes inside another.
'note' becomes 'n' (abbreviation).
'bug' placed around 'n' is 'BUNG'.
'getting' is the link.
(Other definitions for bung that I've seen before include "Stopper for closing hole in barrel" , "Plug for a hole in a small boat" , "Stopper, plug" , "Rubber stopper" , "Tip, bribe (sl.)" .)