Everyone in Slough is degenerate (6)
I believe the answer is:
fallen
'is degenerate' is the definition.
'everyone in slough' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'in' means one lot of letters goes inside another.
'slough' becomes 'fen' (I've seen this before).
'all' inserted into 'fen' is 'FALLEN'.
(Other definitions for fallen that I've seen before include "Having yielded to temptation, or toppled" , "Having sinned or surrendered" , "... _____!" (II Samuel)" , "Down to earth" , "Immoral" .)