A tune that's very good and never ends (3)
I believe the answer is:
air
'a tune' is the definition.
(air can mean a piece of music)
'very good and never ends' is the wordplay.
'very good' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'and' is a charade indicator (letters next to each other).
'ends' says to take the final letters.
The final letter of 'never' is 'r'.
'ai'+'r'='AIR'
'that's' is the link.
(Other definitions for air that I've seen before include "Ventilate or give expression to" , "Of which 78% is nitrogen" , "Ventilate with breathing gas" , "Broadcast j" , "Spread publicity" .)