Formal wear for men in clubs? (4)
'formal wear' is the definition.
The definition and answer can be both states of affairs as well as being singular nouns.
Maybe you can see a link between them that I can't see?
I can't explain the rest of the clue.
(Other definitions for dark that I've seen before include "With little or no light" , "lacking lights" , "Lacking light" , "Sombre in colour" , "Gloomy" .)
This clue is a double definition.
'formal wear for men' is the first definition.
'clubs?' is the second definition.
(in playing cards clubs are a black suit)
'in' acts as a link.