The times when Erasmus didn't get a sum wrong (4)
I believe the answer is:
eras
'the times' is the definition.
(an era is a time or period)
'when erasmus didn't get a sum wrong' is the wordplay.
I cannot really see how this works, but
'when' could be 'as' (eg 'when I walked' means 'as I walked') and 'as' is found within the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
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" .)