Change people in advertisement (5)
I believe the answer is:
'change' is the definition.
(amending is a kind of changing)
'people in advertisement' is the wordplay.
'people' becomes 'men' (men can refer to humanity as a whole).
'in' is an insertion indicator.
'advertisement' becomes 'ad' (synonyms).
'men' placed into '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)" .)