A fellow blond's business (6)
I believe the answer is:
affair
'business' is the definition.
(I've seen this in another clue)
'a fellow blond's' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'blond' becomes 'fair' (similar in meaning).
'a'+'f'+'fair'='AFFAIR'
(Other definitions for affair that I've seen before include "Secret liaison" , "A fling (or something more serious?)" , "Extramarital activity" , "Involvement" , "Romantic relationship, perhaps illicit" .)