Fell, turning over in space (4)
I believe the answer is:
room
'space' is the definition.
(room is a kind of space)
'fell turning over' is the wordplay.
'fell' becomes 'moor' (I've seen this before).
'turning over' is a reversal indicator.
'moor' in reverse letter order is 'ROOM'.
'in' acts as a link.
(Other definitions for room that I've seen before include "Scope; living space" , "Enclosed space in a building" , "Adequate space" , "See 17" , "bedsit?" .)