Less forgiving
There are a few possibilities for this:
stricter
'less forgiving' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sterner that I've seen before include "Renters of more strict sort" , "More demanding" , "More grimly serious" , "more inflexible anyway" , "Less easy-going" .)
(Other definitions for harsher that I've seen before include "More abrasive" , "Rougher or more strict" .)
'less forgiving' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for stricter that I've seen before include "More rigorous" , "Sterner or more unyielding" .)