Material in river (5)
I believe the answer is:
tweed
I believe this clue is a double definition.
'material' is the first definition.
(I know that tweed is a type of material)
'river' is the second definition.
(river in Scotland)
'in' acts as a link.
(Other definitions for tweed that I've seen before include "rough stuff" , "Fabric; river" , "Flower on border" , "Thick woollen cloth - northern river" , "Flow" .)