Monsters back among the grosser goblins (5)
I believe the answer is:
ogres
'monsters' is the definition.
(ogre is a type of monster)
'back among the grosser goblins' is the wordplay.
'back' says the letters should be written in reverse.
'among' indicates the answer is hidden within the clue.
'sergo' can be found hidden inside 'grosser goblins'.
'sergo' in reverse letter order is 'OGRES'.
(Other definitions for ogres that I've seen before include "Giants in fairytales" , "Monstrous giants and man-eaters" , "Formidable people" , "Beasts" , "Gorse (anag.)" .)