Effect of Independent politician over Bill (6)
I believe the answer is:
Here is my best explanation:
'effect' is the definition.
(impact is a kind of effect)
'independent politician over bill' is the subsidiary indication.
'independent' becomes 'i'.
'politician' becomes 'mp'.
'over' means one lot of letters go next to another.
'bill' becomes 'act' (I've seen this in other clues).
'of' is the link.
(Other definitions for impact that I've seen before include "The striking of one body against another", "Repercussion", "Force of blow or collision", "Influence", "hit, in a big way", "The force of one body against another", "Result", "crash", "Serious effect" and "Force exerted by collision".)