It's a blow for one MP to go to law (6)
I believe the answer is:
impact
'it's a blow' is the definition.
(thesaurus)
'one mp to go to law' is the wordplay.
'one' becomes 'i' (Roman numeral).
'to go to' says to put letters next to each other.
'law' becomes 'act' (I've seen this before).
'i'+'mp'+'act'='IMPACT'
'for' is the link.
(Other definitions for impact that I've seen before include "Forceful contact between two objects" , "Serious effect" , "The striking of one thing against another" , "Bear upon" , "Repercussions" .)