Set of letters found in post, ultimately (6)
I believe the answer is:
vowels
'set of letters' is the definition.
'vowels' can be an answer for 'letters' (vowel is a kind of letter). I am not sure about the 'set of' bit.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for vowels that I've seen before include "Speech sounds made with the vocal tract open" , "'Five letters, not consonants (6)'" , "aeiou" , "A e i o u" , "Five speech sounds" .)