Clean western tree (4)
I believe the answer is:
wash
'clean' is the definition.
(washing is a kind of cleaning)
'western tree' is the wordplay.
'western' becomes 'w'.
'tree' becomes 'ash tree' (ash is a kind of tree).
'w'+'ash'='WASH'
(Other definitions for wash that I've seen before include "Bathe" , "Be credible" , "Lave" , "shower" , "Bear investigation" .)