A monster to make go back again (4)
I believe the answer is:
ogre
'a monster' is the definition.
('ogre' can be a synonym of 'monster')
'go back again' is the wordplay.
'back' shows that the letters should be reversed in order.
'again' becomes 're'.
'go' back-to-front is 'og'.
'og'+'re'='OGRE'
'to make' is the link.
(Other definitions for ogre that I've seen before include "Bugbear" , "terrible person" , "Giant of folklore who ate humans" , "Monstrous creature" , "Giant who eats people" .)