Fell back in space (4)
I believe the answer is:
room
'space' is the definition.
(room is a kind of space)
'fell back' is the wordplay.
'fell' becomes 'moor' (I've seen this before).
'back' shows that the letters should be reversed in order.
'moor' written backwards gives 'ROOM'.
'in' acts as a link.
(Other definitions for room that I've seen before include "Space, area" , "Capacity" , "Space (for elbows?)" , "Box ---, storage area" , "See 1" .)