Change people in advertisement (5)
I believe the answer is:
amend
'change' is the definition.
(amending is a kind of changing)
'people in advertisement' is the wordplay.
'people' becomes 'men' (men are people).
'in' indicates putting letters inside.
'advertisement' becomes 'ad' (abbreviation).
'men' inserted inside 'ad' is 'AMEND'.
(Other definitions for amend that I've seen before include "make an improvement" , "Alter in writing" , "Correct error, make minor alteration as to writing" , "Make small changes to" , "Make minor improvements to (something)" .)