Hell of a deep pit (5)
I believe the answer is:
abyss
I believe this clue is a double definition.
'hell' is the first definition.
(I know that hell can be written as abyss)
'deep pit' is the second definition.
(I've seen this before)
'of a' acts as a link.
(Other definitions for abyss that I've seen before include "Seemingly bottomless chasm" , "Primal chaos" , "depths of sea" , "something rather deep" , "Very deep gorge" .)