One fanatic seizing power makes contribution (5)
I believe the answer is:
input
'contribution' is the definition.
(I've seen this before)
'one fanatic seizing power' is the wordplay.
'one' becomes 'i' (Roman numeral).
'fanatic' becomes 'nut' (I've seen this before).
'seizing' indicates putting letters inside.
'power' becomes 'p'.
'nut' placed around 'p' is 'nput'.
'i'+'nput'='INPUT'
'makes' acts as a link.
(Other definitions for input that I've seen before include "supply" , "contributed information" , "Contributory remark(s)" , "Insertion of data" , "Contribution; enter (data)" .)