Old Magistrates, unusually severe (6)
I believe the answer is:
'old magistrates' is the definition.
(I've seen this before)
'unusually severe' is the wordplay.
'unusually' indicates anagramming the letters.
'severe' anagrammed gives 'REEVES'.
(Other definitions for reeves that I've seen before include "Keanu - - (films)" , "officials" , "Old magistrates" .)