Tree of south-western river in Germany (8)
I believe the answer is:
tamarind
'tree' is the definition.
(tamarind tree is a kind of tree)
'river in germany' is the wordplay.
'river' becomes 'Tamar' (river in the southwest of England).
'germany' becomes 'd' (vehicle registration code for Deutschland).
'tamar'+'in'+'d'='TAMARIND'
'of south-western' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tamarind that I've seen before include "Tree providing Asian flavouring" , "Large tropical seedpod with tangy pulp -- mad train (anag)" , "Indian tree with long brown seed pods" , "Seed pod pulp eaten fresh or cooked with rice and fish or preserved in curries and chutneys" , "Tropical African tree" .)