A or E, but not I, O or U (4)
I believe the answer is:
note
'a or e but not i o or u' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for note that I've seen before include "Reminder - fame - sound" , "Observe in Eton" , "Write down" , "Short message; piano key" , "Fiver, say" .)