One fanatic accepting penny contribution (5)
I believe the answer is:
input
'contribution' is the definition.
(I've seen this before)
'one fanatic accepting penny' is the wordplay.
'one' becomes 'i' (Roman numeral).
'fanatic' becomes 'nut' (I've seen this before).
'accepting' is an insertion indicator.
'penny' becomes 'p'.
'nut' going around 'p' is 'nput'.
'i'+'nput'='INPUT'
(Other definitions for input that I've seen before include "Contribution; enter (data)" , "Simple, without artifice" , "Signal supplied to a device" , "Contributory remark(s)" , "Insertion of data" .)