First-rate rook in sky (3)
I believe the answer is:
air
'sky' is the definition.
(I've seen this before)
'first-rate rook' is the wordplay.
'first-rate' becomes 'ai' (resembles A1, informal term for excellent).
'rook' becomes 'r' (abbreviation in chess).
'ai'+'r'='AIR'
'in' acts as a link.
(Other definitions for air that I've seen before include "Song; ventilate" , "Space above and around" , "'Tune, melody (3)'" , "Refrain" , "distinctive quality" .)