Remains of an oleaceous tree? (3)
I believe the answer is:
ash
This clue is a double definition.
'remains' is the first definition.
(remains of a fire)
'an oleaceous tree?' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for ash that I've seen before include ". . . . [FOREST] One of which" , "Tree; fire residue" , "Rowan tree is also called the mountain . . ." , "Fraxinus" , "Hardwood" .)