Y-fronts of yuppies threaten to rip if unwisely manspread (7)
I believe the answer is:
yttrium
'y' is the definition.
(Y is the chemical symbol for the element yttrium)
'fronts of yuppies threaten to rip if unwisely manspread' is the wordplay.
'fronts of' indicates taking the first letters (I've seen 'front of' mean this).
The first letters of 'yuppies threaten to rip if unwisely manspread' is 'YTTRIUM'.
(Other definitions for yttrium that I've seen before include "Metallic element, symbol Y" , "12 [ELEMENT]" , "Table item" , "Chemical element; um, try it (anag.)" , "Rare earth element" .)