Reduce by 50 percent (5)
I believe the answer is:
halve
'reduce by 50 percent' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for halve that I've seen before include "Divide into two equal parts" , "Reduce by a factor of two" , "Divide into two equal pieces" , "Cut down substantially" , "make 10 5 say?" .)