Cut that 3, when split and reassembled (5)
I believe the answer is:
sever
'cut' is the definition.
(severing is a kind of cutting)
I cannot understand how the remainder of the clue works.
(Other definitions for sever that I've seen before include "Detach" , "Cut off suddenly" , "Cut apart, maybe a relationship" , "Put an end to (connection or relationship)" , "Divide by cutting" .)