Power to stop one fanatic's contribution (5)
I believe the answer is:
input
'contribution' is the definition.
(input means someone's contribution to something)
'power to stop one fanatic's' is the wordplay.
'power' becomes 'p' (symbol in physics).
'to stop' indicates putting letters inside.
'one' becomes 'I' (Roman numeral).
'fanatic' becomes 'nut' (I've seen this in another clue).
'i'+'nut'='inut'
'p' inserted inside 'inut' is 'INPUT'.
(Other definitions for input that I've seen before include "store" , "Signal entering a circuit" , "contributed information" , "Signal fed to equipment" , "Contribution of work or information" .)