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' is a charade indicator (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" .)