Unduly severe (5)
I believe the answer is:
harsh
'unduly severe' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for harsh that I've seen before include "Unpleasantly rough" , "'Severe, rough (5)'" , "Unpleasantly stern" , "Not relenting" , "Severe and disagreeable" .)