One parliamentarian, potential mischief-maker? (3)
I believe the answer is:
imp
'potential mischief-maker?' is the definition.
(imaginary mischievous creature)
'one parliamentarian' is the wordplay.
'one' becomes 'i' (Roman numeral).
'parliamentarian' becomes 'MP' (Member of Parliament).
'i'+'mp'='IMP'
(Other definitions for imp that I've seen before include "Scallywag" , "Small mischievous spirit" , "Mischievous creature" , "Little troublemaker" , "Mischievous child" .)