A pair of females ventilate business (6)
I believe the answer is:
affair
'business' is the definition.
(I've seen this before)
'a pair of females ventilate' is the wordplay.
'pair of' indicates the doubling of some letters (I've seen 'pair' mean this).
'females' becomes 'f' (this might be a standard abbreviation I've not previously seen).
'ventilate' becomes 'air' (synonyms).
'f' duplicated is 'ff'.
'a'+'ff'+'air'='AFFAIR'
(Other definitions for affair that I've seen before include "Secret liaison" , "See 23" , "Concern,matter" , "Happening" , "A fling (or something more serious?)" .)