Sex club (5)
I believe the answer is:
union
I believe this is a double definition.
'sex' is the first definition.
(union is a kind of sex)
'club' is the second definition.
(thesaurus)
(Other definitions for union that I've seen before include "match" , "Agreement -- wedlock" , "Big old pearl" , "Trade organisation to represent workers" , "Climbing plant" .)