Simple tune not bad but lacking introduction (3)
I believe the answer is:
air
'simple tune' is the definition.
(air can mean a piece of music)
'not bad but lacking introduction' is the wordplay.
'not bad' becomes 'fair' (fair can mean of decent quality).
'but lacking introduction' means to remove the first letter.
'fair' with its initial letter taken off is 'AIR'.
(Other definitions for air that I've seen before include "in which birds fly around" , "that's inspired" , "Short song" , "Display" , "Region above the ground" .)