Cut down the middle (5)
(Other definitions for halve that I've seen before include "Reduce by a factor of two" , "Divide fifty-fifty" , "Divide equally between two" , "Divide into two equal parts" , "Cut down substantially" .)
(Other definitions for split that I've seen before include "Divide - Croatian port" , "Cleave lengthwise" , "Cut, cleft" , "Mediterranean port" , "resort on the Adriatic" .)