Work hard to get a new advertising phrase (6)
I believe the answer is:
slogan
'advertising phrase' is the definition.
(I've seen this in another clue)
'work hard to get a new' is the wordplay.
'work hard' becomes 'slog' (slog can mean to work very hard).
'to get' is a charade indicator (letters next to each other).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'slog'+'a'+'n'='SLOGAN'
(Other definitions for slogan that I've seen before include "Ad. catchphrase" , "Shibboleth" , "Identifying catchword" , "Catchy phrase in advertisement etc" , "short political statement" .)