The effect of one member on legislation (6)
I believe the answer is:
impact
'the effect' is the definition.
(impact is a kind of effect)
'one member on legislation' is the wordplay.
'one' becomes 'i' (Roman numeral).
'member' becomes 'MP' (Member of Parliament).
'on' says to put letters next to each other.
'legislation' becomes 'act' (I've seen this in another clue).
'i'+'mp'+'act'='IMPACT'
'of' is the link.
(Other definitions for impact that I've seen before include "The force fo violent strike" , "Force of blow or collision" , "hit, in a big way" , "Forceful striking" , "Impression" .)