Club in underworld is corrupt (5)
I believe the answer is:
disco
'club' is the definition.
(night club)
'in underworld is corrupt' is the wordplay.
'in' indicates a hidden word.
'DISCO' can be found hidden inside 'underworld is corrupt'.
(Other definitions for disco that I've seen before include "Party with records" , "Dance club with recorded music" , "Thumping music" , "One might get down to this" , "20 club" .)