Influence MP to stop one bill (6)
I believe the answer is:
impact
'influence' is the definition.
(I know that impact is a type of influence)
'mp to stop one bill' is the wordplay.
'to stop' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'bill' becomes 'act' (I've seen this in other clues).
'i'+'act'='iact'
'mp' inserted into 'iact' is 'IMPACT'.
(Other definitions for impact that I've seen before include "Force of colllision or blow" , "Force or shock of a collision" , "Crash" , "Serious effect" , "Bear upon" .)