Most strict (8)
I believe the answer is:
sternest
'most strict' is the definition.
('stern' can be a synonym of 'strict')
This is the entire clue.
(Other definitions for sternest that I've seen before include "Most severe and unrelenting" , "Most unfriendly" , "Most strict and demanding" , "Most serious and unrelenting" , "Strictest or grimmest" .)