Cut, thus causing severe reduction (5)
I believe the answer is:
sever
'cut thus' is the definition.
'sever' can be an answer for 'cut' (to sever is to cut or split). I am unsure of the 'thus' bit.
'severe reduction' is the wordplay.
'reduction' means to remove the last letter.
'severe' with its final letter taken off is 'SEVER'.
'causing' acts as a link.
Can you help me to learn more?
(Other definitions for sever that I've seen before include "Detach" , "Lop off" , "Verse (anag)" , "Divide, cut" , "Divorce" .)