Work hard on an advertising catchphrase (6)
I believe the answer is:
slogan
'advertising catchphrase' 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' means one lot of letters go next to another.
'slog'+'an'='SLOGAN'
(Other definitions for slogan that I've seen before include "Word or phrase adopted as motto" , "Marketing phrase" , "Memorable advertising phrase" , "advertising catch phrase" , "Rallying cry" .)