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