O, for one (5)
I believe the answer is:
vowel
'o for one' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for vowel that I've seen before include "Non-consonant" , "E.g. A, but not B" , "Letter representing sound" , "A or e, say" , "A, E, I, O or U" .)