Toil over an advertising expression (6)
I believe the answer is:
slogan
'advertising expression' is the definition.
(a slogan might be used as part of an advert)
'toil over an' is the wordplay.
'toil' becomes 'slog' (I've seen this in another clue).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'slog'+'an'='SLOGAN'
(Other definitions for slogan that I've seen before include "Catchy phrase in advertisement etc" , "Advertising catchphrase" , "Identifying catchword" , "short political statement" , "'Motto, watchword (6)'" .)