Severe, rigorous (6)
I believe the answer is:
strict
This clue is a double definition.
'severe' is the first definition.
(I've seen this before)
'rigorous' is the second definition.
(I know that rigorous can be written as strict)
(Other definitions for strict that I've seen before include "Unbending, precise" , "Rigidly accurate" , "Rigorous, severe" , "Austere - rigid" , "Allowing no laxness" .)