Influence politician supporting one bill (6)
I believe the answer is:
impact
'influence' is the definition.
(I know that impact is a type of influence)
'politician supporting one bill' is the wordplay.
'politician' becomes 'MP' (Member of Parliament).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'one' becomes 'i' (Roman numeral).
'bill' becomes 'act' (I've seen this in other clues).
'mp' after 'i' is 'imp'.
'imp'+'act'='IMPACT'
(Other definitions for impact that I've seen before include "Forcible contact; marked influence" , "Significance" , "Bear upon" , "Collision - effect" , "The striking of one body against another" .)