What the advertisement says is “Work to an end” (6)
I believe the answer is:
slogan
'what the advertisement says' is the definition.
I can't tell whether this definition defines the answer.
'work to an end' is the wordplay.
'work' becomes 'slog' (I've seen this before).
'to an' becomes 'a'.
'end' becomes 'n' (I can't explain this - if you can you should give a lot more credence to this answer).
'slog'+'a'+'n'='SLOGAN'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for slogan that I've seen before include "Short catchy phrase used in advertising" , "Ad. catchphrase" , "Tag line" , "Catchy phrase in advertisement etc" , "Memorable advertising phrase" .)