Horse passes round wide tree (5)
I believe the answer is:
rowan
'tree' is the definition.
(type of tree)
'horse passes round wide' is the wordplay.
'horse passes' becomes 'roan' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'round' indicates putting letters inside.
'wide' becomes 'w' (abbreviation e.g. in cricket).
'roan' placed around 'w' is 'ROWAN'.
Can you help me to learn more?
(Other definitions for rowan that I've seen before include "Canterbury" , "Sort of ash" , "Atkinson or Williams" , "Tree - archbishop" , "Red-berried tree" .)