Effect of bill introduced by one MP (6)
I believe the answer is:
Here is my best explanation:
'effect' is the definition.
(impact is a kind of effect)
'bill introduced by one mp' is the subsidiary indication.
'bill' becomes 'act' (I've seen this in other clues).
'introduced by' means one lot of letters go next to another.
'one' becomes 'i' (roman numeral).
'act' after 'imp' is 'impact'.
'of' is the link.
(Other definitions for impact that I've seen before include "Repercussion", "Effectiveness", "Hit; effect", "Repercussions", "The force fo violent strike", "Result", "Forceful contact between two objects", "Force or shock of a collision", "Significance" and "Serious effect".)