Sound like a faulty chainsaw, cutting out first in … (5)
I believe the answer is:
schwa
'sound' is the definition.
(I know that schwa is a type of mid vowel)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for schwa that I've seen before include "Uh' sound" , "Indistinct sound" , "Stress-free sound" , "sound unstressed" , "Unstressed vowel (phonetics)" .)