First-class rook in sky (3)
I believe the answer is:
air
'sky' is the definition.
(I've seen this in another clue)
'first-class rook' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'rook' becomes 'R' (chess abbreviation).
'ai'+'r'='AIR'
'in' acts as a link.
(Other definitions for air that I've seen before include "Skye Boat Song, perhaps" , "Ventilate, give voice to" , "Broadcast; express" , "Tune; mien" , "The earth's atmosphere" .)