Tie up (boat) (4)
I believe the answer is:
moor
'tie up boat' is the definition.
(to moor a boat is to tie it to land)
This is all the clue.
(Other definitions for moor that I've seen before include "Tract of uncultivated upland" , "Securely fix" , "Open land covered with heather and bracken" , "Tract of open upland" , "eg Ilkley, Othello" .)