All dead in tree? (5)
I believe the answer is:
olive
'tree?' is the definition.
(olive is a kind of tree)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for olive that I've seen before include "Source of edible oil" , "Type of green" , "Oil-producing fruit" , "Evergreen Mediterranean tree" , "Tree - shade of green" .)