Much more than decimate! (5)
I believe the answer is:
halve
'decimate' is the definition.
Although both the answer and definition are verbs in their base form, I don't see how one could define the other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for halve that I've seen before include "make 10 5 say?" , "cut by fifty percent" , "split 50/50" , "Bisect" , "Reduce by 50%" .)