Small tree pessimistic French writer's climbing (5)
I believe the answer is:
sumac
'small tree' is the definition.
(I've seen this in another clue)
'pessimistic french writer's climbing' is the wordplay.
'pessimistic french writer' becomes 'Camus' (existentialist writer Albert Camus).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'camus' reversed gives 'SUMAC'.
(Other definitions for sumac that I've seen before include "Tree colourful in autumn" , "Garden shrub" , "Subtropical tree" , "Hairy-fruited rhus" , "purchase from garden centre?" .)