Four, five, all regularly seen in college (5)
I believe the answer is:
oriel
'college' is the definition.
(Oxford University college)
'four five all regularly seen' is the wordplay.
'regularly seen' indicates alternate letters.
'four'+'five'+'all'='fourfiveall'
The alternating letters of 'fourfiveall' are 'ORIEL'.
'in' is the link.
(Other definitions for oriel that I've seen before include "Large snake" , "Opening" , "Projecting window in upper storey" , "Upper-story bay window" , "Form of bay window" .)