Leant on the tree and collapsed (10)
I believe the answer is:
threatened
'leant' is the definition.
The definition and answer can be both to do with fundamental existence as well as being past participle verbs.
Maybe you can see a link between them that I can't see?
'the tree and collapsed' is the wordplay.
'collapsed' is an anagram indicator.
'the'+'tree'+'and'='thetreeand'
'thetreeand' with letters rearranged gives 'THREATENED'.
'on' is the link.
Can you help me to learn more?
(Other definitions for threatened that I've seen before include "Put at risk, endangered" , "was imminent" , "Menaced" , "Facing danger" , "potentially facing extinction" .)