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