Space saver at last, with low back (4)
I believe the answer is:
room
'space' is the definition.
(room is a kind of space)
'saver at last with low back' is the wordplay.
'at last' indicates one should take the final letters.
'with' is a charade indicator (letters next to each other).
'low' becomes 'moo' ('moo' can be a synonym of 'low').
'back' shows that the letters should be reversed in order.
The final letter of 'saver' is 'r'.
'moo' reversed gives 'oom'.
'r'+'oom'='ROOM'
(Other definitions for room that I've seen before include "basic accommodation" , "Space within a building" , "Enclosed indoor space" , "Capacity" , "Space; part of building" .)