Boss appearing in the advertisement (4)
I believe the answer is:
head
'boss' is the definition.
('head' can be a synonym of 'boss')
'in the advertisement' is the wordplay.
'in' indicates a hidden word.
'HEAD' is hidden within 'the advertisement'.
'appearing' is the link.
(Other definitions for head that I've seen before include "Promontory; foam" , "Chief; go first" , "Chief; part of body" , "''and still the wonder grew / That one small ... could carry all he knew'' (Goldsmith)" , "Chief; promontory" .)