A tune-up during the opera (4)
I believe the answer is:
aria
'during the opera' is the definition.
I can't tell whether this definition defines the answer.
'a tune-up' is the wordplay.
'tune' becomes 'air' (air can mean a piece of music).
'up' is a reversal indicator (in down clue: letters go upwards).
'air' backwards is 'ria'.
'a'+'ria'='ARIA'
Can you help me to learn more?
(Other definitions for aria that I've seen before include "Song from an opera (4)" , "Opera solo" , "Operatic solo performance" , "Air in an opera" , "Imitative art work" .)