Having good ventilation, like a first-class railway (4)
I believe the answer is:
airy
'having good ventilation' is the definition.
I can't judge whether this definition defines the answer.
'first-class railway' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'railway' becomes 'ry' (abbreviation for railway).
'ai'+'ry'='AIRY'
'like a' is the link.
Can you help me to learn more?
(Other definitions for airy that I've seen before include "Well-ventilated; delicate" , "Well ventilated; nonchalant" , "open to the sky" , "Light and insubstantial" , "(Of ideas) speculative" .)