Supernatural being eating startled gnu: swallow completely (6)
I believe the answer is:
engulf
'swallow completely' is the definition.
(I've seen this before)
'supernatural being eating startled gnu' is the wordplay.
'supernatural being' becomes 'elf' (elf is a kind of supernatural being).
'eating' means one lot of letters goes inside another.
'startled' indicates anagramming the letters.
'gnu' anagrammed gives 'ngu'.
'elf' going around 'ngu' is 'ENGULF'.
(Other definitions for engulf that I've seen before include "Completely overwhelm" , "Swallow completely" , "Surround or cover completely" , "Bury" , "Completely swamp" .)