Rook flies south from farm on old moor (6)
I believe the answer is:
anchor
'moor' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
(Other definitions for anchor that I've seen before include "ground" , "Be moored" , "Horse-breeding farm" , "Last runner in relay race team" , "Brake for a boat" .)