Effect of bill introduced by one MP (6)
I believe the answer is:
impact
'effect' is the definition.
(impact is a kind of effect)
'bill introduced by one mp' is the wordplay.
'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).
'i'+'mp'='imp'
'act' after 'imp' is 'IMPACT'.
'of' is the link.
(Other definitions for impact that I've seen before include "The striking of one body against another" , "Significance" , "Forceful striking" , "Forcible contact between two things" , "Collision - effect" .)