Mean woman in bad temper, blowing top? (7)
I believe the answer is:
Here is my best explanation:
'mean' is the definition.
('average' can be a synonym of 'mean')
'woman in bad temper blowing top?' is the subsidiary indication.
'woman' becomes 'vera' (a woman's name**).
'in' is an insertion indicator.
'bad temper' becomes 'rage'.
'blowing top?' means to remove the first letter.
'rage' with its first letter taken away is 'age'.
'vera' put inside 'age' is 'average'.
(Other definitions for average that I've seen before include "Mean figure?", "Moderate", "It may be mean", "'Medium, middling (7)'", "Just about normal", "The statistical norm", "Mean - everyday", "It's common", "Usual - mediocre - mean" and "Mean, median or middling".)