Unpleasantly stern (5)
I believe the answer is:
harsh
'unpleasantly stern' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for harsh that I've seen before include "Strict, unfair" , "Not relenting" , "Severe; bleak" , "Raucous - austere - bitter" , "Jarring" .)