Group of nations thus abandons song: it backed oil producers (9)
I believe the answer is:
eucalypti
'oil producers' is the definition.
(I've seen this before)
'group of nations thus abandons song it backed' is the wordplay.
I cannot quite see how this works, but
'group' could be 'uca' (Uca is an example) and 'uca' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for eucalypti that I've seen before include "Gum trees" , "Gums" , "Oil sources" , "Oil producers" , "shrubs" .)