Rigorous, exacting (6)
I believe the answer is:
strict
I believe this clue is a double definition.
'rigorous' is the first definition.
(I know that rigorous can be written as strict)
'exacting' is the second definition.
(I know that exacting can be written as strict)
(Other definitions for strict that I've seen before include "Exacting, unsparing" , "Undeviating" , "Observing rules very closely" , "Rigidly enforced" , "Rigorous, uncompromising" .)