Start to chop tree without a top (5)
I believe the answer is:
crown
'a top' is the definition.
(crown is a kind of top)
I can't explain the rest of the clue.
(Other definitions for crown that I've seen before include "Visible part of a tooth" , "piece of tooth" , "Hit on the head (informal)" , "Diadem" , "'Uneasy lies the head that wears a . . . . .'" .)