Space to tie up when returning (4)
I believe the answer is:
room
'space' is the definition.
(enough space or room to do something)
'tie up when returning' is the wordplay.
'tie up' becomes 'moor' (as in mooring a boat).
'when returning' shows that the letters should be reversed in order.
'moor' in reverse letter order is 'ROOM'.
'to' acts as a link.
(Other definitions for room that I've seen before include "bedsit?" , "Enclosed part of building" , "Enclosed space in a building" , "Scope; living space" , "Opportunity, scope" .)