Fairly insignificant Republican is imprisoned (6)
I believe the answer is:
pretty
'fairly' is the definition.
(thesaurus)
'insignificant republican is imprisoned' is the wordplay.
'insignificant' becomes 'petty' (I've seen this before).
'republican' becomes 'r' (abbreviation).
'is imprisoned' means one lot of letters goes inside another.
'petty' placed around 'r' is 'PRETTY'.
(Other definitions for pretty that I've seen before include "Fairly - good-looking" , "like a kettle of fish" , "Film with Julia Roberts (belle)" , "Pleasing on the eye" , "Delicately attractive" .)