Passing by six leading actresses (3)
I believe the answer is:
via
'passing by' is the definition.
(thesaurus)
'six leading actresses' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'leading' suggests taking the first letters (the leading letter of the word).
The initial letter of 'actresses' is 'a'.
'vi'+'a'='VIA'
(Other definitions for via that I've seen before include "Passing through" , "Going through" , "Roman road" , "By way of - in Latin" , "By means of" .)