Work hard on an advertising phrase (6)
I believe the answer is:
slogan
'advertising phrase' is the definition.
(I've seen this before)
'work hard on an' is the wordplay.
'work hard' becomes 'slog' (I've seen this before**).
'on' is a charade indicator (letters next to each other).
'slog'+'an'='SLOGAN'
(Other definitions for slogan that I've seen before include "Identifying catchword" , "Tag line" , "Battle cry - advertising phrase" , "Word or phrase adopted as motto" , "Short catchy phrase used in advertising" .)