Business relationship (6)
I believe the answer is:
affair
I believe this is a double definition.
'business' is the first definition.
(I've seen this before)
'relationship' is the second definition.
(I know that affair is a type of sexual relationship)
(Other definitions for affair that I've seen before include "See 23" , "scandal" , "Liaison or intrigue" , "Secret sexual relationship" , "Romantic relationship, perhaps illicit" .)