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' means one lot of letters goes inside another.
'wood' becomes 'copse' (a copse is a small wood).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'all' backwards is 'lla'.
'copse' placed around '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" .)