The "Y" of YSL (4)
I believe the answer is:
yves
'the y of ysl' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for yves that I've seen before include "-- Saint-Laurent, couturier" , "Montand, Fr. actor" , "Frenchman maybe" , "French male name" , "-- Saint-Laurent, French fashion designer" .)