Relaxed beside an underworld figure (5)
I believe the answer is:
satan
'underworld figure' is the definition.
(I've seen this in another clue)
'relaxed beside an' is the wordplay.
'relaxed' becomes 'sat' (sitting is a type of relaxing).
'beside' says to put letters next to each other (I've seen this in other clues).
'sat'+'an'='SATAN'
(Other definitions for satan that I've seen before include "Prince of Darkness" , "Apollyon" , "The devil, Beelzebub" , "Diabolical figure" , "Fiend" .)