One politician is a mischief-maker (3)
I believe the answer is:
imp
'mischief-maker' is the definition.
(mischievous creature)
'one politician' is the wordplay.
'one' becomes 'i' (Roman numeral).
'politician' becomes 'MP' (Member of Parliament).
'i'+'mp'='IMP'
'is a' is the link.
(Other definitions for imp that I've seen before include "Goblin" , "Mischievous monkey" , "Small mischievous sprite" , "Elf" , "Minx" .)