College that's left behind alternative (5)
I believe the answer is:
oriel
'college' is the definition.
(Oxford University college)
'that's left behind alternative' is the wordplay.
'that's' becomes 'ie'.
'left' becomes 'l' (common abbreviation).
'behind' is a charade indicator (letters next to each other).
'alternative' becomes 'or'.
'ie'+'l'='iel'
'iel' after 'or' is 'ORIEL'.
(Other definitions for oriel that I've seen before include "Type of window; Oxford college" , "Loire (anag.)" , "English college" , "Recessed bay window" , "polygonal recess" .)