Unstressed vowel we analysed originally after school (5)
I believe the answer is:
schwa
'unstressed vowel' is the definition.
(I've seen this in another clue)
'we analysed originally after school' is the wordplay.
'originally' indicates taking the first letters.
'after' is a charade indicator (letters next to each other).
'school' becomes 'sch' (common abbreviation).
The first letters of 'we analysed' is 'wa'.
'wa' put after 'sch' is 'SCHWA'.
(Other definitions for schwa that I've seen before include "Uh' sound" , "Stress-free sound" , "sound unstressed" , "Indistinct sound" , "Unstressed vowel (phonetics)" .)