In the wood, climbing, all fall down (8)
I believe the answer is:
collapse
'fall down' is the definition.
(I've seen this before)
'in the wood climbing all' is the wordplay.
'in the' is an insertion indicator.
'wood' becomes 'copse' (a copse is a small wood).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'all' backwards is 'lla'.
'copse' enclosing 'lla' is 'COLLAPSE'.
(Other definitions for collapse that I've seen before include "Fall down or lose strength suddenly" , "Give way or cave in" , "Suddenly fall down or give way" , "Faint and fall" , "Disintegration" .)