Hole in one rare tree (4)
I believe the answer is:
'tree' is the definition.
(I've seen this before)
'hole in one rare' is the wordplay.
'hole in one' becomes 'ace' (I've seen this before).
'rare' becomes 'r' (listed by Oxford Dictionary of Abbreviations).
(Other definitions for acer that I've seen before include "ornamental tree", "Type of maple", "Tree of maple family", "Tree with winged fruit", "Care about this tree".)