Hell, now he’s coming round! (5)
I believe the answer is:
hades
'hell' is the definition.
('Hades' can be a synonym of 'hell')
'now he's coming round' is the wordplay.
'now' becomes 'ad' (I've seen this in other clues).
'he' becomes 'hes' (I can't explain this - if you can you should give a lot more credence to this answer).
'coming round' indicates putting letters inside.
'ad' inserted inside 'hes' is 'HADES'.
Can you help me to learn more?
(Other definitions for hades that I've seen before include "gloomy place" , "In Greek myth, the Underworld" , "Inferno" , "It's classical hell in the shade" , "The underworld where the souls of the dead live" .)