Tree in delta below river (8)
I believe the answer is:
tamarind
'tree' is the definition.
(tamarind is a kind of tree)
'in delta below river' is the wordplay.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'below' says to put letters next to each other (in a down clue, some letters go below others).
'river' becomes 'tamar' (I've seen this before).
'in'+'d'='ind'
'ind' put after 'tamar' is 'TAMARIND'.
(Other definitions for tamarind that I've seen before include "Tropical African tree" , "Large tropical seedpod with tangy pulp -- mad train (anag)" , "exotic fruit" , "Tree providing Asian flavouring" , "Indian tree with long brown seed pods" .)