Passing light reflected over grass (8)
I believe the answer is:
elapsing
'passing' is the definition.
('elapse' can be a synonym of 'pass')
'light reflected over grass' is the wordplay.
'light reflected' becomes 'pale' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'over' says the letters should be written in reverse.
'grass' becomes 'sing' (both can mean to act as an informant).
'pale' in reverse letter order is 'elap'.
'elap'+'sing'='ELAPSING'
Can you help me to learn more?
(Another definition for elapsing that I've seen is " Passing, as time".)