Work hard over an advertising catchphrase (6)
I believe the answer is:
slogan
'advertising catchphrase' is the definition.
(I've seen this before)
'work hard over an' is the wordplay.
'work hard' becomes 'slog' (slog can mean to work very hard).
'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 "short political statement" , "Ad. catchphrase" , "Shibboleth" , "Identifying catchword" , "War cry - advertising jingle" .)