Gigantic mummy, creature of the night (7)
I believe the answer is:
mammoth
'gigantic' is the definition.
(I know that gigantic can be written as mammoth)
'mummy creature of the night' is the wordplay.
'mummy' becomes 'mam' (both can mean a mother).
'creature of the night' becomes 'moth' (most moths are nocturnal).
'mam'+'moth'='MAMMOTH'
(Other definitions for mammoth that I've seen before include "Large extinct beast" , "Enormous" , "Huge" , "Gigantic - extinct animal" , "Extinct type of elephant" .)