Harsh or very strict (6)
I believe the answer is:
severe
I believe this clue is a double definition.
'harsh' is the first definition.
(I've seen this in another clue)
'very strict' is the second definition.
(I've seen this before)
(Other definitions for severe that I've seen before include "Austere, spartan" , "Plain, hard" , "Drastic" , "Extremely difficult to endure" , "Terrible" .)