A western wood is flooded (5)
I believe the answer is:
awash
'is flooded' is the definition.
(synonyms)
'a western wood' is the wordplay.
'western' becomes 'w'.
'wood' becomes 'ash' (wood from an ash tree).
'a'+'w'+'ash'='AWASH'
(Other definitions for awash that I've seen before include "not quite under" , "With the top just covered with water" , "becomes flooded" , "Partly under water" , "Flood" .)