Excellent rook in sky (3)
I believe the answer is:
air
'sky' is the definition.
(I've seen this before)
'excellent rook' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'rook' becomes 'R' (chess abbreviation).
'ai'+'r'='AIR'
'in' is the link.
(Other definitions for air that I've seen before include "Refrain" , "Musical composition" , "An impressive manner or tune" , "Means of travelling" , "Region above the ground" .)