Assistant rejects official scoring error (5)
I believe the answer is:
gofer
'assistant' is the definition.
(I have seen 'Menial assistant ' mean 'gofer' so perhaps 'assistant' could also mean 'gofer')
'rejects official scoring error' is the wordplay.
'rejects' says the letters should be written backwards.
'official' becomes 'ref' (match official in football, say).
'scoring error' becomes 'og' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ref'+'og'='refog'
'refog' back-to-front is 'GOFER'.
Can you help me to learn more?
(Other definitions for gofer that I've seen before include "caddie?" , "Informal word for one who runs errands" , "Menial assistant" , "Dogsbody" , "Errand runner (colloq.)" .)