Tree that's almost mature associated with Christian festival (8)
I believe the answer is:
oleaster
'tree' is the definition.
(I've seen this before)
'almost mature associated with christian festival' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'mature' becomes 'old' ('old' is associated in meaning with 'mature').
'associated with' means one lot of letters go next to another.
'christian festival' becomes 'easter' (I've seen this before).
'old' with its last letter removed is 'ol'.
'ol'+'easter'='OLEASTER'
'that's' acts as a link.
(Other definitions for oleaster that I've seen before include "Tree" , "One produces fruit" , "Plant" , "Shrub; steel oar (anag.)" , "Shrub like an olive" .)