Horse circling western tree (5)
I believe the answer is:
rowan
'tree' is the definition.
(type of tree)
'horse circling western' is the wordplay.
'horse' becomes 'roan' (roan is a kind of horse).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'western' becomes 'w'.
'roan' going around 'w' is 'ROWAN'.
(Other definitions for rowan that I've seen before include "Tree - archbishop" , "Mountain-ash" , "Canterbury" , "Sort of ash" , "Red-berried tree" .)