One fanatic accepts penny contribution (5)
I believe the answer is:
input
'contribution' is the definition.
(input means someone's contribution to something)
'one fanatic accepts penny' is the wordplay.
'one' becomes 'i' (Roman numeral).
'fanatic' becomes 'nut' (I've seen this before).
'accepts' indicates putting letters inside.
'penny' becomes 'p' (short for pence).
'nut' placed around 'p' is 'nput'.
'i'+'nput'='INPUT'
(Other definitions for input that I've seen before include "Contribution - data fed to computer" , "Signal feeding a circuit" , "Signal supplied to a device" , "Insertion of data" , "Comment, contribution" .)