Crack, split (4)
I believe the answer is:
rift
This clue is a double definition.
'crack' is the first definition.
(I know that rift is a type of crack)
'split' is the second definition.
(I've seen this in other clues)
(Other definitions for rift that I've seen before include "Serious falling out" , "Breach - falling-out" , "A disagreement or an earth cleft" , "Crack, split, break" , "Break in relations; type of valley" .)