Rather small-minded about Rex (6)
I believe the answer is:
pretty
'rather' is the definition.
('pretty' can be a synonym of 'rather')
'small-minded about rex' is the wordplay.
'small-minded' becomes 'petty' ('petty' can be a synonym of 'small-minded').
'about' means one lot of letters goes inside another.
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'petty' enclosing 'r' is 'PRETTY'.
(Other definitions for pretty that I've seen before include "Attractive, beautiful" , "sweet" , "Fairly; appealing" , "Attractive - quite" , "Pleasing on the eye" .)