The most common and most likely answer to this crossword clue is the 9 letter word MANNA-ASH
I've seen this crossword clue in Independent
M
A
N
N
A
-
A
S
H
More clues leading to the result MANNA-ASH
Buddy explains!
The straight part
We know that "18 (i.e.tree)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 18 (i.e.tree) = manna-ash.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.