Fantastic creatures one's cowed by nonetheless (5)
I believe the answer is:
yetis
'fantastic creatures' is the definition.
'yetis' can be an answer for 'creatures' (I've seen this before). I'm not sure about the 'fantastic' bit.
'one's cowed by nonetheless' is the wordplay.
I cannot quite understand how this works, but
'one's' could be 'is' (Roman numeral) and 'is' is located in the answer.
'nonetheless' could be 'yet' (I've seen this in another clue) and 'yet' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for yetis that I've seen before include "More than one mythic creature" , "Mythical creatures" , "Mountain monsters" , "Abominable snowmen" , "Beasts that aren't real" .)