To the left, space to 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)
'to the left space' is the wordplay.
'to the left' says the letters should be written backwards (reading from right to left).
'space' becomes 'room' (room is a kind of space).
'room' reversed gives 'MOOR'.
'to' is the link.
(Other definitions for moor that I've seen before include "Berth, tie up" , "Land mainly high" , "Use ropes to secure" , "Make a boat secure - wasteland - Mauritanian" , "Fasten" .)