Pressure to stop one head's contribution (5)
I believe the answer is:
input
'contribution' is the definition.
(input means someone's contribution to something)
'pressure to stop one head's' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'to stop' is an insertion indicator (stop can mean to plug a hole in something).
'one' becomes 'i' (Roman numeral).
'head' becomes 'nut' (informal term for somebody's head).
'i'+'nut'='inut'
'p' going within 'inut' is 'INPUT'.
(Other definitions for input that I've seen before include "Contribution of work or information" , "Data etc supplied to computer" , "Signal entering a circuit" , "Data entered to computer" , "Comment, contribution" .)