Conclude working in retirement is hell (7)
I believe the answer is:
inferno
'is hell' is the definition.
(I've seen this before)
'conclude working in retirement' is the wordplay.
'conclude' becomes 'infer' (inferring is a kind of concluding).
'working' becomes 'on'.
'in retirement' says the letters should be written backwards (retire can mean to go back or backwards).
'on' reversed gives 'no'.
'infer'+'no'='INFERNO'
(Other definitions for inferno that I've seen before include "blazer?" , "Large dangerous fire" , "Huge conflagration" , "Huge fire" , "(Hellish) blaze" .)