Rich drunk found under British tree (5)
I believe the answer is:
birch
'tree' is the definition.
(birch is a kind of tree)
'rich drunk found under british' is the wordplay.
'drunk' indicates an anagram.
'found under' is a charade indicator (letters next to each other).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'rich' anagrammed gives 'irch'.
'irch' after 'b' is 'BIRCH'.
(Other definitions for birch that I've seen before include "Silver-white barked tree" , "Flog" , "Tree with grey or white bark" , "'Slender tree, the bark could be silver (5)'" , "to beat the rest?" .)