Bad boy out of bounds climbing tree (5)
I believe the answer is:
olive
'tree' is the definition.
(I know that olive is a type of tree)
'bad boy out of bounds climbing' is the wordplay.
'bad' becomes 'evil' (similar in meaning).
'out of bounds' means to look at the middle letters (the bounds or edges of the word are removed).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
The middle letter of 'boy' is 'o'.
'evil'+'o'='evilo'
'evilo' written backwards gives 'OLIVE'.
(Other definitions for olive that I've seen before include "shades of green" , "Fruit also used to make oil" , "Yellow-green colour like Mediterranean fruit" , "Female" , "Small green Mediterranean tree fruit, nice in martini" .)