Excellent singer finally in tune (3)
I believe the answer is:
air
'tune' is the definition.
(air can mean a piece of music)
'excellent singer finally' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'finally' indicates one should take the final letters.
The final letter of 'singer' is 'r'.
'ai'+'r'='AIR'
'in' is the link.
(Other definitions for air that I've seen before include "coming through the open window" , "Gas mix surrounding the earth" , "14" , "Ventilate with oxygen" , "character" .)