Window that is in airport, unclosed (5)
I believe the answer is:
oriel
'window that is in airport unclosed' is the definition.
'oriel' can be an answer for 'window' (oriel is a type of window). I am not sure about the remainder of the definition.
'window that is in' is the wordplay.
'window' becomes 'orl' (I can't explain this - if you can you should believe this answer much more).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'in' means one lot of letters goes inside another.
'orl' enclosing 'ie' is 'ORIEL'.
Can you help me to learn more?
(Other definitions for oriel that I've seen before include "Recessed bay window" , "Protruding window" , "Form of bay window" , "Upstairs bay window" , "Type of bay window" .)