Wood or metal initially placed in river (6)
I believe the answer is:
timber
'wood' is the definition.
(I know that timber is a type of wood)
'metal initially placed in river' is the wordplay.
'initially' suggests taking the first letters.
'placed in' means one lot of letters goes inside another (I've seen this in other clues).
'river' becomes 'tiber' (River Tiber in Italy).
The initial letter of 'metal' is 'm'.
'm' placed into 'tiber' is 'TIMBER'.
'or' is the link.
(Other definitions for timber that I've seen before include "Watch out for falling tree!" , "The tree's falling!" , "Lumber" , "Wood that's been cut and prepared" , "Lumberjack's warning call" .)