Excuse average teacher (6)
I believe the answer is:
pardon
'excuse' is the definition.
('pardon' can be a synonym of 'excuse')
'average teacher' is the wordplay.
'average' becomes 'par' (I've seen this before).
'teacher' becomes 'don' (don is a kind of teacher).
'par'+'don'='PARDON'
(Other definitions for pardon that I've seen before include ""I'm sorry, could you say that again?"" , "Forgive - eh?" , "Forgive or excuse (an offence)" , "Excuse or absolve" , "Forgive (an error)" .)