Social organisations for which a black suit is required (5)
I believe the answer is:
clubs
'social organisations' is the definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.
'a black suit is required' is the wordplay.
'a black suit' becomes 'club'.
'is required' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'club'+'s'='CLUBS'
'for which' acts as a link.
Can you help me to learn more?
(Other definitions for clubs that I've seen before include "Orders" , "Associations of people with similar interests" , "For game, you may need five" , "cabarets" , "discos" .)