Space to anchor round (4)
I believe the answer is:
room
'space' is the definition.
(enough space or room to do something)
'anchor round' is the wordplay.
'anchor' becomes 'moor' (I've seen this before).
'round' says the letters should be written in reverse.
'moor' in reverse letter order is 'ROOM'.
'to' acts as a link.
(Other definitions for room that I've seen before include "Capacity" , "basic accommodation" , "Adequate space" , "Enclosed part of building" , "See 1" .)