Monsters in bog resurfaced (5)
I believe the answer is:
ogres
'monsters' is the definition.
('ogre' can be a synonym of 'monster')
'in bog resurfaced' is the wordplay.
'in' indicates a hidden word.
'OGRES' can be found hidden inside 'bog resurfaced'.
(Other definitions for ogres that I've seen before include "Bugbears" , "Awful brutes" , "Gorse (anag.)" , "Monstrous giants and man-eaters" , "Giant types" .)