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