Conclude that it’s hell without a negative (5)
I believe the answer is:
infer
'conclude' is the definition.
(inferring is a kind of concluding)
'it's hell without a negative' is the wordplay.
'it's hell' becomes 'inferno' ('inferno' can be a synonym of 'hell').
'without' is a deletion indicator.
'a negative' becomes 'no' (no is a kind of negative).
'inferno' with 'no' taken out is 'INFER'.
'that' acts as a link.
(Other definitions for infer that I've seen before include "come to a conclusion" , "Read between the line" , "Work out" , "Insinuate" , "Draw a conclusion" .)