Severe — not entirely Mister Nice (5)
I believe the answer is:
stern
'severe' is the definition.
(I know that severe can be written as stern)
'not entirely mister nice' is the wordplay.
'not entirely' says the answer is hidden in the clue.
'STERN' is hidden in the letters of 'mister nice'.
(Other definitions for stern that I've seen before include "Back end of a ship" , "Austere rear part of a ship?" , "Rents out at rear of boat" , "Rigorous - back of boat" , "Stiff - back - not smiling" .)