Hard work, many discovered, to produce advertising jingle (6)
I believe the answer is:
slogan
'advertising jingle' is the definition.
(I've seen this before)
'hard work many discovered' is the wordplay.
'hard work' becomes 'slog' (a slog is a long or difficult task).
'discovered' means to look at the middle letters (dis-covered - outer letters removed).
The middle letters of 'many' are 'an'.
'slog'+'an'='SLOGAN'
'to produce' is the link.
(Other definitions for slogan that I've seen before include "short political statement" , "War cry - advertising jingle" , "Catchy phrase in advertisement etc" , "Tag line" , "Motto, catchy phrase" .)