Crossword Genius

Cut that 3, when split and reassembled (5)

Ross

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" .)

I've seen this clue in The Guardian.
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