Characters with flair are really not so common (5)
I believe the answer is:
rarer
'not so common' is the definition.
('rare' can be the opposite of 'common' so perhaps 'rarer' is reasonable for 'not so common')
'characters with flair are really' is the wordplay.
'characters' says the answer is hidden in the clue (some letters or characters from the clue).
'RARER' can be found hidden inside 'with flair are really'.
(Other definitions for rarer that I've seen before include "Comparatively sparse" , "Thinner" , "More scarce or uncommon" , "less easy to find" , "Stranger" .)