Race round tree (4)
I believe the answer is:
acer
'tree' is the definition.
(acer is a type of tree)
'race round' is the wordplay.
'round' is an anagram indicator.
'race' anagrammed gives 'ACER'.
(Other definitions for acer that I've seen before include "Plant" , "Tree of maple family" , "Type of maple" , "Care about the maple?" , "Tree with winged fruit" .)