Light, first-class railway (4)
I believe the answer is:
airy
'light' is the definition.
(I've seen this before)
'first-class railway' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'railway' becomes 'ry' (abbreviation for railway).
'ai'+'ry'='AIRY'
(Other definitions for airy that I've seen before include "Well-ventilated; delicate" , "Delicate; dismissive" , "Sprightly" , "Spacious; nonchalant" , "Well ventilated; insubstantial" .)