Tree in great danger, ultimately (4)
I believe the answer is:
acer
'tree' is the definition.
(I've seen this in other clues)
'great danger ultimately' is the wordplay.
'great' becomes 'ace' (ace can informally mean very good).
'ultimately' indicates one should take the final letters.
The last letter of 'danger' is 'r'.
'ace'+'r'='ACER'
'in' acts as a link.
(Other definitions for acer that I've seen before include "Type of maple" , "Plant" , "Tree with winged fruit" , "Tree or shrub of the maple genus" , "Care about the maple?" .)