Effect of independent member introducing law (6)
I believe the answer is:
Here is my best explanation:
'effect' is the definition.
(impact is a kind of effect)
'independent member introducing law' is the subsidiary indication.
'independent' becomes 'i'.
'member' becomes 'mp'.
'introducing' is a charade indicator (letters next to each other).
'law' becomes 'act' (I've seen this in other clues).
'of' is the link.
(Other definitions for impact that I've seen before include "A big hit", "Repercussions", "Forceful consequence", "Result", "Effectiveness", "crash", "Force of blow or collision", "The striking of one body against another", "Forceful contact between two objects" and "Effect - collision".)