Malign ring around tree (5)
I believe the answer is:
olive
'tree' is the definition.
(I know that olive is a type of tree)
'malign ring around' is the wordplay.
'malign' becomes 'evil' (synonyms).
'ring' becomes 'O' (O is the shape of a ring).
'around' says the letters should be written in reverse.
'evil'+'o'='evilo'
'evilo' backwards is 'OLIVE'.
(Other definitions for olive that I've seen before include "Green shade; its branch for peace" , "Stoned fruit" , "A Med. fruit" , "This branch is offered in peace gesture" , "yellowish green" .)