Fissure, chasm (4)
I believe the answer is:
rift
I believe this clue is a double definition.
'fissure' is the first definition.
(I know that rift is a type of fissure)
'chasm' is the second definition.
(I've seen this in another clue)
(Other definitions for rift that I've seen before include "gap" , "Breach - falling-out" , "Serious falling out" , "Breach or quarrel" , "Cleft or fissure" .)