The times when Erasmus didn't get a sum wrong (4)
I believe the answer is:
eras
'the times' is the definition.
(era is a kind of time)
'erasmus didn't get a sum wrong' is the wordplay.
I cannot quite see how this works, but
'a' is found in the answer.
The remaining letters 'ers' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
'when' is the link.
Can you help me to learn more?
(Other definitions for eras that I've seen before include "typeface" , "Epochs" , "Long and distinct periods of history" , "Periods of time" , "Periods, ages" .)