Ball of thread (4)
I believe the answer is:
clew
'ball of thread' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for clew that I've seen before include "Mayo bay overlooked by Croagh Patrick" , "Ball of twine" , "old ball of thread" , "Corner of a ship's sail and Mayo bay" , "Piece of canvas" .)