Fall back, everyone, to hide among trees (8)
I believe the answer is:
collapse
'fall' is the definition.
(collapse can mean to fall down)
'back everyone to hide among trees' is the wordplay.
'back' says the letters should be written in reverse.
'everyone' becomes 'all' (all people).
'to hide among' indicates putting letters inside.
'trees' becomes 'copse' (a copse is a small wood).
'all' backwards is 'lla'.
'lla' inserted inside 'copse' is 'COLLAPSE'.
(Other definitions for collapse that I've seen before include "Give way or cave in" , "Give way, fail suddenly" , "downfall" , "Faint and fall" , "Fall or disintergrate" .)