Contribution from one head assuming power (5)
I believe the answer is:
input
'contribution' is the definition.
(I've seen this before)
'one head assuming power' is the wordplay.
'one' becomes 'i' (Roman numeral).
'head' becomes 'nut' (informal term for somebody's head).
'assuming' indicates putting letters inside.
'power' becomes 'p'.
'nut' enclosing 'p' is 'nput'.
'i'+'nput'='INPUT'
'from' acts as a link.
(Other definitions for input that I've seen before include "Signal supplied to a device" , "Feed data into a computer" , "Comment, contribution" , "Person's contribution (to a project)" , "Data entry" .)