Bottomless gulf (5)
I believe the answer is:
abyss
'bottomless gulf' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for abyss that I've seen before include "Deep hole, seems bottomless" , "something rather deep" , "Seemingly bottomless chasm" , "Hell; hole" , "Primal chaos" .)