Roman god of the underworld (3)
I believe the answer is:
dis
'roman god of the underworld' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dis that I've seen before include "Treat without respect - god of the underworld" , "Speak disrespectfully to (colloq.)" , "Pluto's underworld" , "Greek underworld" , "The god Pluto" .)