Correction — approval by millions in depression (9)
I believe the answer is:
amendment
'correction' is the definition.
(amendment is a kind of correction)
'approval by millions in depression' is the wordplay.
'approval' becomes 'amen' (amen to that).
'by' is a charade indicator (letters next to each other).
'millions' becomes 'm' (abbreviation).
'in' is an insertion indicator.
'depression' becomes 'dent' (dent is a kind of depression).
'm' placed within 'dent' is 'dment'.
'amen'+'dment'='AMENDMENT'
(Other definitions for amendment that I've seen before include "change for the better" , "Addition or alteration, to a motion or official document, say" , "Change designed to improve a text" , "Correction" , "Minor improvement" .)