'Work hard!' -- a new rallying cry (6)
I believe the answer is:
slogan
'rallying cry' is the definition.
(thesaurus)
'work hard a new' is the wordplay.
'work hard' becomes 'slog' (I've seen this before**).
'a new' becomes 'an' (I've seen this in other clues).
'slog'+'an'='SLOGAN'
(Other definitions for slogan that I've seen before include "Motto, catchy phrase" , "Shibboleth" , "short political statement" , "Memorable advertising phrase" , "Advertising catchphrase" .)