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' indicates putting letters inside (circling can mean going around or containing).
'western' becomes 'w'.
'roan' placed 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" .)