Dirty look threatening after nightclub (9)
I believe the answer is:
discolour
'dirty' is the definition.
(to make discoloured)
'look threatening after nightclub' is the wordplay.
'look threatening' becomes 'lour' (to lour is to stare menacingly).
'after' is a charade indicator (letters next to each other).
'nightclub' becomes 'disco' (I've seen this before).
'lour' put after 'disco' is 'DISCOLOUR'.
(Other definitions for discolour that I've seen before include "Fade - stain" , "fox?" , "tarnished" , "remove shades" .)