Horse going round western tree (5)
I believe the answer is:
rowan
'tree' is the definition.
(type of tree)
'horse going round western' is the wordplay.
'horse' becomes 'roan' (roan is a kind of horse).
'going round' indicates putting letters inside (some letters go around others).
'western' becomes 'w'.
'roan' placed around 'w' is 'ROWAN'.
(Other definitions for rowan that I've seen before include "source of berries" , "Red-berried tree" , "Mountain-ash" , "Tree - archbishop" , "Atkinson or Williams" .)