Space around this heath (4)
I believe the answer is:
moor
'this heath' is the definition.
(thesaurus)
'space around' is the wordplay.
'space' becomes 'room' (room is a kind of space).
'around' shows that the letters should be reversed in order.
'room' reversed gives 'MOOR'.
(Other definitions for moor that I've seen before include "Heather-covered tract" , "Expanse of uncultivated upland" , "Desolate area" , "'On Ilka - - baht'at'" , "eg Ilkley, Othello" .)