Work hard with an advertising jingle (6)
I believe the answer is:
slogan
'advertising jingle' is the definition.
(I've seen this before)
'work hard with an' is the wordplay.
'work hard' becomes 'slog' (slog can mean to work very hard).
'with' is a charade indicator (letters next to each other).
'slog'+'an'='SLOGAN'
(Other definitions for slogan that I've seen before include "'Motto, watchword (6)'" , "short political statement" , "Memorable advertising phrase" , "Logo" , "Catchy phrase in advertisement etc" .)