Dated pupil, once only, note (8)
I believe the answer is:
obsolete
'dated' is the definition.
(I've seen this in another clue)
'pupil once only note' is the wordplay.
'pupil once' becomes 'ob'.
'only' becomes 'sole' (synonyms).
'note' becomes 'te' (musical note).
'ob'+'sole'+'te'='OBSOLETE'
(Other definitions for obsolete that I've seen before include "Obese lot may be out of date" , "Pass" , "No longer useful" , "Boots eel away, being out of date" , "Outdated" .)