Space to tie a ship up (4)
I believe the answer is:
room
'space' is the definition.
(enough space or room to do something)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for room that I've seen before include "Scope; living space" , "Place" , "See 1" , "Capacity" , "Enclosed indoor space" .)