Hard work, an advertising phrase (6)
I believe the answer is:
slogan
'advertising phrase' is the definition.
(I've seen this before)
'hard work an' is the wordplay.
'hard work' becomes 'slog' (a slog is a long or difficult task).
'slog'+'an'='SLOGAN'
(Other definitions for slogan that I've seen before include "Tag line" , "War cry - advertising jingle" , "Identifying catchword" , "Catchy phrase in advertisement etc" , "Logo" .)