Glacial fissure (8)
I believe the answer is:
crevasse
'glacial fissure' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for crevasse that I've seen before include "opening" , "Deep glacial crack" , "gap" , "Deep fissure in a glacier" , "Deep open crack as in glacier" .)