Advertisement showing dog crossing lake (4)
I believe the answer is:
plug
'advertisement' is the definition.
(thesaurus)
'dog crossing lake' is the wordplay.
'dog' becomes 'pug' (pug is a kind of dog**).
'crossing' indicates putting letters inside.
'lake' becomes 'l'.
'pug' enclosing 'l' is 'PLUG'.
'showing' acts as a link.
(Other definitions for plug that I've seen before include "Stopper - advert" , "Mention favourably - stopper" , "Piece of chewing tobacco" , "Good word" , "Wad of tobacco and electrical accessory" .)