Retro company regularly built in circular windows (5)
I believe the answer is:
oculi
'circular windows' is the definition.
(an oculus is a round window)
'retro company regularly built in' is the wordplay.
'retro' is a reversal indicator (retro is Latin for 'backwards').
'company' becomes 'co' (abbreviation).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'built'+'in'='builtin'
The alternate letters of 'builtin' are 'uli'.
'co' in reverse letter order is 'oc'.
'oc'+'uli'='OCULI'
(Other definitions for oculi that I've seen before include "Eyes; eye-like opening in architecture, like a round window" , "Circular windows" .)