Advertisement, fair and correct (6)
I believe the answer is:
adjust
'correct' is the definition.
(adjusting is a kind of correcting)
'advertisement fair' is the wordplay.
'advertisement' becomes 'ad' ('ad' can be a synonym of 'advertisement').
'fair' becomes 'just' ('just' can be a synonym of 'fair').
'ad'+'just'='ADJUST'
'and' is the link.
(Other definitions for adjust that I've seen before include "Make small changes" , "Alter slightly to be right for use" , "Correct" , "Tune" , "Adapt, regulate" .)