Fairly narrow-minded about rule (6)
I believe the answer is:
pretty
'fairly' is the definition.
(thesaurus)
'narrow-minded about rule' is the wordplay.
'narrow-minded' becomes 'petty' ('petty' can be similar in meaning to 'narrow-minded').
'about' means one lot of letters goes inside another.
'rule' becomes 'r' (legal abbreviation).
'petty' going around 'r' is 'PRETTY'.
(Other definitions for pretty that I've seen before include "Fairly - good-looking" , "Delicately attractive" , "chocolate-box" , "Reasonably" , "charming to look at" .)