Structure ultimately supporting crooked palm tree (5)
I believe the answer is:
maple
'tree' is the definition.
(I know that maple is a type of angiospermous tree)
'structure ultimately supporting crooked palm' is the wordplay.
'ultimately' suggests the final letters.
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'crooked' is an anagram indicator (a crooked or disrupted version of the letters).
The last letter of 'structure' is 'e'.
'palm' with letters rearranged gives 'mapl'.
'e' after 'mapl' is 'MAPLE'.
(Other definitions for maple that I've seen before include "Leaves on this" , "Emblematic tree of Canada" , "Tree whose leaf features on the flag of Canada" , "Type of syrup" , "Tree providing Canada's national emblem" .)