Some utter nightmare for flier (4)
I believe the answer is:
tern
'flier' is the definition.
(tern is a kind of birdanimal)
'some utter nightmare for' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'TERN' is hidden within 'utter nightmare for'.
(Other definitions for tern that I've seen before include "flier" , "Long-winged aquatic bird" , "Winger" , "Three-masted schooner" , "Rent out this gull" .)