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 "Effect - collision", "The striking of one thing against another", "Forceful contact between two objects", "Bear upon", "Marked effect or influence", "Repercussion", "Force or shock of a collision", "Forcible contact; marked influence", "Significance" and "Forceful consequence".)