Occupying zoo: great big monster (4)
I believe the answer is:
ogre
'monster' is the definition.
(fairy tale monster)
'occupying zoo great big' is the wordplay.
'occupying' says the answer is hidden in the clue (letters for the answer occupy part of the clue).
'OGRE' is hidden within 'zoo great big'.
(Other definitions for ogre that I've seen before include "Grim(m) beast?" , "'Demon, fiend (4)'" , "terrible person" , "Has a monstrous role" , "stern one who inspires fear" .)