Source of metal behind tree on land (6)
I believe the answer is:
ashore
'on land' is the definition.
('ashore' can be a synonym of 'on land')
'source of metal behind tree' is the wordplay.
'source of metal' becomes 'ore' (I've seen this before).
'behind' is a charade indicator (letters next to each other).
'tree' becomes 'ash' (ash is a kind of tree).
'ore' after 'ash' is 'ASHORE'.
(Other definitions for ashore that I've seen before include "Going on land from sea" , "Beached" , "Toward land from water" , "On(to) dry land" , "Aground" .)