An underworld racket? (4,2,1,5)

'an underworld racket?' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'underworld racket?' is the wordplay.
I cannot really understand how this works, but
'underworld' could be 'hell' (synonyms) and 'hell' is located in the answer.
'racket?' could be 'noise' (racket can mean a loud noise) and 'noise' is present in the answer.
The remaining letters 'ofa' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?