What moon reveals after autumn in Oregon Trail (4,6)

I believe the answer is:

fall behind

'trail' is the definition.
(thesaurus)

'what moon reveals after autumn in oregon' is the wordplay.
'what moon reveals' becomes 'behind' (to moon is to expose one's backside).
'after' is a charade indicator (letters next to each other).
'autumn in oregon' becomes 'fall' (American English for Autumn).
'behind' after 'fall' is 'FALL BEHIND'.

(Another definition for fall behind that I've seen is " Lag".)

I've seen this clue in The Everyman puzzle in The Observer.

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