Most severe (8)
There are a few possibilities for this:
strictest
'most severe' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sternest that I've seen before include "Most severe and unrelenting" , "Most serious and unrelenting" , "Strictest or grimmest" , "Most strict and demanding" , "Most unfriendly" .)
'most severe' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Another definition for harshest that I've seen is " Most severe".)
(Another definition for strictest that I've seen is " Most rigorous".)