One politician is a rogue (3)
I believe the answer is:
imp
'rogue' is the definition.
(thesaurus)
'one politician' is the wordplay.
'one' becomes 'i' (Roman numeral).
'politician' becomes 'MP' (Member of Parliament).
'i'+'mp'='IMP'
'is a' acts as a link.
(Other definitions for imp that I've seen before include "A little devil of mischief" , "Mischievous fairy No.1" , "Goblin" , "Pixie" , "Evil spirit" .)