Out West, a horse or a tree (5)
I believe the answer is:
rowan
'tree' is the definition.
(type of tree)
'out west a horse or a' is the wordplay.
I cannot quite see how this works, but
'west' could be 'w' (abbreviation) and 'w' is found in the answer.
'a' could be 'an' and 'an' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rowan that I've seen before include "Tree bearing bright red berries" , "Man" , "The mountain ash tree" , "Red-berried tree" , "Canterbury" .)