Somewhat minor mistake compared to the average (4)
I believe the answer is:
norm
'the average' is the definition.
('norm' can be a synonym of 'average')
'somewhat minor mistake compared' is the wordplay.
'somewhat' indicates the answer is hidden within the clue.
'NORM' is hidden in the letters of 'minor mistake compared'.
'to' is the link.
(Other definitions for norm that I've seen before include "Usual or standard thing" , "Most frequent state" , "'Standard, usual (4)'" , "Accepted standard of behaviour" , "Average level of achievement" .)