Four? Five? All regularly seen in window (5)
I believe the answer is:
oriel
'window' is the definition.
(oriel is a type of window)
'four? five? all regularly seen' is the wordplay.
'regularly seen' means one should take alternating letters.
'four'+'five'+'all'='fourfiveall'
The alternate letters of 'fourfiveall' are 'ORIEL'.
'in' is the link.
(Other definitions for oriel that I've seen before include "Upstairs bay window" , "corbelled window?" , "Style of window" , "English college" , "Upper room window" .)