Light first-class railway (4)
I believe the answer is:
airy
'light' is the definition.
(synonyms)
'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 "Flippant" , "Lacking substance" , "affecting nonchalance" , "Breezy" , "Spacious - aerial" .)