Plunge into a den of iniquity! (4)
I believe the answer is:
dive
This clue is a double definition.
'plunge' is the first definition.
(I know that plunge can be written as dive)
'a den of iniquity' is the second definition.
(thesaurus)
'into' acts as a link.
(Other definitions for dive that I've seen before include "Jump into water" , "Joint" , "Plunge head first" , "Plunge, taken by some footballers" , "sleazy bar" .)