Having shape of Y-fronts in young people's style is leaving oldies in despair (8)
I believe the answer is:
ypsiloid
'having shape of y' is the definition.
(ypsiloid means shaped like the letter Y)
'fronts in young people's style is leaving oldies in despair' is the wordplay.
'fronts in' indicates taking the first letters (the letters are the front).
'in despair' becomes 'id' (this might be a standard abbreviation I've not previously seen).
The initial letters of 'young peoples style is leaving oldies' is 'ypsilo'.
'ypsilo'+'id'='YPSILOID'
(Other definitions for ypsiloid that I've seen before include "Slipping, spoil DIY" , "Looking like letter from Athens" .)