One parliamentarian is a devil (3)
I believe the answer is:
imp
'a devil' is the definition.
(I've seen this before)
'one parliamentarian' is the wordplay.
'one' becomes 'i' (Roman numeral**).
'parliamentarian' becomes 'mp'.
'i'+'mp'='IMP'
'is' is the link.
(Other definitions for imp that I've seen before include "one likely to cause trouble" , "Elf" , "Little mischief-maker" , "Small mischievous sprite" , "A mischievous little devil" .)