Monster in for cheeseburger (3)
I believe the answer is:
orc
'monster' is the definition.
(fantasy creature)
'in for cheeseburger' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ORC' is hidden in the letters of 'for cheeseburger'.
(Other definitions for orc that I've seen before include "Tolkien goblin" , "Tolkienian monster" , "Sea monster; or Tolkien character" , "Fierce goblin" , "Tolkien ogre" .)