Branch of the UN? (5)
I believe the answer is:
olive
'branch' is the definition.
(I have seen 'Branch of peace ' mean 'olive' so perhaps 'branch' could also mean 'olive')
'of the un?' is the wordplay.
'of' becomes 'o''.
'the un?' becomes 'live' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'o'+'live'='OLIVE'
Can you help me to learn more?
(Other definitions for olive that I've seen before include "Oil-yielding tree" , "Source of edible oil" , "Green shade; its branch for peace" , "Oil-yielding fruit" , "Bitter fruit" .)