Tune with a first-class, rousing start (3)
I believe the answer is:
air
'tune' is the definition.
(air can mean a piece of music)
'with a first-class rousing start' is the wordplay.
'with' means one lot of letters go next to another.
'a first class' becomes 'ai' (A1 - top quality).
'start' indicates taking the first letters.
The first letter of 'rousing' is 'r'.
'ai' put next to 'r' is 'AIR'.
(Other definitions for air that I've seen before include "Element; tune" , "Look - warm and dry" , "Mixture of gases, especially oxygen" , "- - on a G-string" , "An impressive manner or tune" .)