Slowly and deliberately work on an advertising phrase (6)

I believe the answer is:

slogan

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'advertising phrase' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for slogan that I've seen before include "Rallying cry" , "Motto, catchy phrase" , "advertising catch phrase" , "Marketing phrase" , "Identifying catchword" .)

I've seen this clue in The Telegraph.

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