They are defrauded of a penny in fees (5)
I believe the answer is:
dupes
'they are defrauded' is the definition.
The definition suggests an adverb but the answer is not.
'a penny in fees' is the wordplay.
'a penny' becomes 'p' (short for pence).
'in' means one lot of letters goes inside another.
'fees' becomes 'dues' (payments which are due).
'p' inserted inside 'dues' is 'DUPES'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for dupes that I've seen before include "Gulls" , "Fools, hoaxes" , "Cheats" , "Hoodwinks" , "Fools, deceives" .)