Excellent railway, well ventilated (4)
I believe the answer is:
airy
'well ventilated' is the definition.
(letting a lot of air through)
'excellent railway' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'railway' becomes 'ry' (abbreviation for railway).
'ai'+'ry'='AIRY'
(Other definitions for airy that I've seen before include "Fanciful" , "affecting nonchalance" , "Light, delicate" , "(Of ideas) speculative" , "Well ventilated; insubstantial" .)