Some older referees returning made mistakes (5)
I believe the answer is:
erred
'made mistakes' is the definition.
(I've seen this in another clue)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for erred that I've seen before include "Was mistaken" , "Went astray" , "Made mistakes" , "Sinned" , "Committed a sin" .)