Noble having sex with French philosopher (7)
I believe the answer is:
Here is my best explanation:
'noble' is the definition.
(I've seen this before)
'sex with french philosopher' is the wordplay.
'sex' becomes 'vi' ('sex' is Latin for 'six' or VI in Roman numerals).
'with' is a charade indicator (letters next to each other).
'french philosopher' becomes 'comte' (I've seen this before. I am not sure about the 'french' bit.**).
'having' acts as a link.
(Other definitions for vicomte that I've seen before include "Foreign nobleman", "French nobleman" and "Noble".)