The most common and most likely answer to this crossword clue is the 8 letter word OLEASTER
I've seen this crossword clue in Independent
O
L
E
A
S
T
E
R
More clues leading to the result OLEASTER
- Wild olive tree also going up the wall
- Let a rose become untidy shrub
- Plant, centrepiece of holy Christian festival
- Plant almost mature by Resurrection time
- Tree that’s almost mature associated with Christian festival
- Plant parts central to Holy Christian festival
- Olive Oyl forgets Popeye’s 5th anniversary
- Shrub has nothing left by the daffodil season?
- Tree, also wild olive
- After expression of approval, plant fruit-bearing shrub
Buddy explains!

The straight part
We know that "T/r/ee" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case t/r/ee = oleaster.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does OLEASTER mean?
OLEASTER noun- A plant in the family Elaeagnaceae,
- Cultivated olive trees that have re-naturalized, sometimes treated as a species Olea oleaster, the wild olive.