To cut or reduce severely (5)
I believe the answer is:
sever
'to cut' is the definition.
(to sever is to cut or split)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for sever that I've seen before include "Cut off suddenly" , "Divorce" , "Divide, cut" , "Cut completely" , "Put an end to (connection or relationship)" .)