Cave in wood holding everyone up (8)
I believe the answer is:
collapse
'cave' is the definition.
(I've seen this in another clue)
'wood holding everyone up' is the wordplay.
'wood' becomes 'copse' (a copse is a small wood).
'holding' is an insertion indicator.
'everyone' becomes 'all' (all people).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'all' reversed gives 'lla'.
'copse' placed around 'lla' is 'COLLAPSE'.
'in' acts as a link.
(Other definitions for collapse that I've seen before include "Sudden fall" , "fold" , "Fall or disintergrate" , "Disintegration" , "Sudden breakdown" .)