Work hard on a new catchphrase (6)
I believe the answer is:
slogan
'catchphrase' is the definition.
(I've seen this before)
'work hard on a new' is the wordplay.
'work' becomes 'slog' (I've seen this before**).
'hard' is an anagram indicator.
'on' means one lot of letters go next to another.
'new' becomes 'n'.
'a' put after 'n' is 'na'.
'na' with letters rearranged gives 'an'.
'slog'+'an'='SLOGAN'
(Other definitions for slogan that I've seen before include "Battle cry - advertising phrase" , "War cry - advertising jingle" , "Word or phrase adopted as motto" , "Short catchy phrase used in advertising" , "advertising catch phrase" .)