Passing white's soaring over grass (8)

'passing' is the definition.
('elapse' can be a synonym of 'pass')

'white's soaring over grass' is the wordplay.
'white' becomes 'pale' (I've seen this before).
'soaring' says the letters should be written backwards (in a down clue, letters 'soar' up).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'grass' becomes 'sing' (both can mean to act as an informant).
'pale' backwards is 'elap'.
'elap'+'sing'='ELAPSING'