Tree, a sure thing to be cut down (4)
I believe the answer is:
acer
'tree' is the definition.
(I've seen this before)
'a sure thing to be cut down' is the wordplay.
'sure thing' becomes 'cert' (I've seen this before).
'to be cut down' means to remove the last letter.
'cert' with its last letter taken off is 'cer'.
'a'+'cer'='ACER'
(Other definitions for acer that I've seen before include "Type of maple" , "Tree with winged fruit" , "Care about this tree" , "Tree or shrub of the maple genus" , "Tree of maple family" .)