Effect of Independent politician over Bill (6)
I believe the answer is:
impact
'effect' is the definition.
(impact is a kind of effect)
'independent politician over bill' is the wordplay.
'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**).
'i'+'mp'+'act'='IMPACT'
'of' is the link.
(Other definitions for impact that I've seen before include "Forceful contact between two objects" , "Repercussion" , "Serious effect" , "Strong effect" , "The striking of one body against another" .)