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 wordplay.
'speech' becomes 'oration' (oration is a kind of speech).
'in the course of which' means one lot of letters goes inside another.
'cleared out' means to remove the middle letters.
'people' with its middle taken out is 'pe'.
'oration' enclosing 'pe' is 'OPERATION'.
(Other definitions for operation that I've seen before include "act", "Procedure; enterprise", "management", "surgical procedure", "running", "Manoeuvre - military action", "Working", "Surgery", "To one pair (anag.)" and "working".)