Class in good time with no answer, for once (8)
I believe the answer is:
formerly
'once' is the definition.
('formerly' can be a synonym of 'once')
'class in good time with no answer' is the wordplay.
'class' becomes 'form' (form class is a kind of class).
'in good time' becomes 'early' (being early is arriving in good time).
'with no' indicates named letters should be taken away.
'answer' becomes 'a' (abbreviation eg in Q and A).
'early' with 'a' taken away is 'erly'.
'form'+'erly'='FORMERLY'
'for' is the link.
(Other definitions for formerly that I've seen before include "Previously, before this time" , "In the past" , "Hitherto" , "At one time" .)