One helps pupils losing way in Windows (5)
I believe the answer is:
oculi
'windows' is the definition.
(plural of oculus, an eye-like opening)
'one helps pupils losing way' is the wordplay.
'one helps pupils' becomes 'oculist' (an oculist might treat one's pupils).
'losing' indicates named letters should be taken away.
'way' becomes 'st' (abbreviation for street).
'oculist' with 'st' taken away is 'OCULI'.
'in' is the link.
(Other definitions for oculi that I've seen before include "Eyes; eye-like opening in architecture, like a round window" , "Circular windows" .)