Just one MP and a rigged trial (9)
I believe the answer is:
impartial
'just' is the definition.
(associated in meaning)
'one mp and a rigged trial' is the wordplay.
'one' becomes 'i' (Roman numeral).
'and' says to put letters next to each other.
'rigged' is an anagram indicator.
'trial' anagrammed gives 'rtial'.
'i'+'mp'+'a'+'rtial'='IMPARTIAL'
(Other definitions for impartial that I've seen before include "Unbiased" , "Objective" , "Fair, unprejudiced" , "not taking sides" , "Disinterested" .)