Hm, there isn't one! (5)
I believe the answer is:
vowel
'hm' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for vowel that I've seen before include "aeio or u" , "Open sound in speech" , "E.g. A, but not B" , "Not a consonant" , "A, E, I, O or U" .)