Rather narrow-minded about head of religion (6)
I believe the answer is:
pretty
'rather' is the definition.
('pretty' can be a synonym of 'rather')
'narrow-minded about head of religion' is the wordplay.
'narrow-minded' becomes 'petty' ('petty' can be similar in meaning to 'narrow-minded').
'about' is an insertion indicator.
'head of' suggests taking the first letters.
The initial letter of 'religion' is 'r'.
'petty' going around 'r' is 'PRETTY'.
(Other definitions for pretty that I've seen before include "Delicately attractive" , "sweet" , "charming to look at" , "comely" , "See 19" .)