Campaign speech in the course of which people cleared out (9)
I believe the answer is:
Here is my best explanation:
'campaign' is the definition.
'speech in the course of which people cleared out' is the subsidiary indication.
'speech' becomes 'oration' (oration is a kind of speech**).
'in the course of which' is an insertion indicator.
'cleared out' means to remove the middle letters.
'people' with its centre taken out is 'pe'.
'oration' enclosing 'pe' is 'operation'.
(Other definitions for operation that I've seen before include "Procedure; enterprise", "Surgery", "Surgical or military procedure", "Surgical or military piece of work", "Manoeuvre - military action", "Act of surgery", "To one pair (anag.)", "Theatre procedure" and "Action; process".)