Inflexible about rules (6)
I believe the answer is:
strict
'inflexible about rules' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for strict that I've seen before include "Requiring exact obedience or compliance with rules" , "Authoritarian" , "Following rules exactly" , "Rigorous, severe" , "No-nonsense" .)