Rigorous; severe (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)
'severe' is the second definition.
(I've seen this in another clue)
(Other definitions for strict that I've seen before include "Not allowing laxity" , "Uncompromising in discipline" , "Allowing no deviation from a standard" , "Unbending, precise" , "Exact, rigorous" .)