Metal-worker with tree (6)
I believe the answer is:
welder
'tree' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'metal-worker' is the wordplay.
'metal' becomes 'W' (W is an example).
'worker' becomes 'elder' (elder is a kind of worker).
'w'+'elder'='WELDER'
'with' acts as a link.
(Other definitions for welder that I've seen before include "Metal joiner" , "metalworker" , "Tradesman who fuses metal using heat" , "Torch-bearer" , "Skilled worker who fuses metals using heat" .)