Republican period brought up? Not common (4)
I believe the answer is:
rare
'not common' is the definition.
('rare' is the opposite to 'common')
'republican period brought up?' is the wordplay.
'republican' becomes 'r' (abbreviation).
'period' becomes 'era' (era is a kind of period**).
'brought up?' is a reversal indicator (down clues only).
'era' reversed gives 'are'.
'r'+'are'='RARE'
(Other definitions for rare that I've seen before include "few and far between" , "Scarce as hen's teeth!" , "Seldom - unusual" , "'Unusual, uncommon (4)'" , "Might steak be" .)