Everyone back in thicket is faint (8)
I believe the answer is:
collapse
'faint' is the definition.
(thesaurus)
'everyone back in thicket' is the wordplay.
'everyone' becomes 'all' (all people).
'back' says the letters should be written backwards.
'in' is an insertion indicator.
'thicket' becomes 'copse' (synonyms).
'all' reversed gives 'lla'.
'lla' inserted inside 'copse' is 'COLLAPSE'.
'is' acts as a link.
(Other definitions for collapse that I've seen before include "Faint and fall" , "Fall or disintergrate" , "Sudden breakdown" , "Abrupt failure of function" , "Sudden fall" .)