Tree research site backed by South Africa (5)
I believe the answer is:
balsa
'tree' is the definition.
(balsa is a kind of tree)
'research site backed by south africa' is the wordplay.
'research site' becomes 'lab' (I've seen this in another clue).
'backed' is a reversal indicator.
'by' is a charade indicator (letters next to each other).
'south africa' becomes 'SA' (abbreviation).
'lab' back-to-front is 'bal'.
'bal'+'sa'='BALSA'
(Other definitions for balsa that I've seen before include "Tropical American tree" , "Very lightweight wood from tropical tree" , "A slab (anag.)" , "South American tree" , "light raft" .)