Light rising piece of music gliding away (8)
I believe the answer is:
elapsing
'light rising piece of music gliding away' is the definition.
I can't tell whether this definition defines the answer.
'light rising piece of music gliding' is the wordplay.
'light' becomes 'pale' (similar in meaning).
'rising' says the letters should be written in reverse (going up in a down clue).
'piece of music gliding' becomes 'sing' (I am not sure about this - if you are sure you should believe this answer much more).
'pale' backwards is 'elap'.
'elap'+'sing'='ELAPSING'
Can you help me to learn more?
(Another definition for elapsing that I've seen is " Passing, as time".)