Descriptive statistic revised where data give incomplete information (8,7)
I believe the answer is:
'descriptive statistic' is the definition.
(I know that Weighted average is a type of average)
'revised where data give incomplete information' is the wordplay.
'revised' indicates an anagram.
'incomplete' means to remove the last letter.
'information' becomes 'gen' (gen can informally mean information).
'gen' with its last letter removed is 'ge'.
'wheredatagive' with letters rearranged gives 'weightedavera'.