River skirting the end of Birnam Wood (6)
I believe the answer is:
timber
'wood' is the definition.
(I've seen this before)
'river skirting the end of birnam' is the wordplay.
'river' becomes 'tiber' (River Tiber in Italy).
'skirting' means one lot of letters goes inside another.
'the end of' suggests the final letters.
The final letter of 'birnam' is 'm'.
'tiber' placed around 'm' is 'TIMBER'.
(Other definitions for timber that I've seen before include "Trees suitable for building or carpentry" , "Watch out for falling tree!" , "Feller's warning cry" , "Wood that's been cut and prepared" , "The tree's falling!" .)