The most common and most likely answer to this crossword clue is the 2 plus 2 plus 4 plus 2 plus 2 plus 3 letter word GO TO WORK ON AN EGG
I've seen this crossword clue in The Daily Telegraph
G
O
T
O
W
O
R
K
O
N
A
N
E
G
G
More clues leading to the result GO TO WORK ON AN EGG
Buddy explains!
The straight part
We know that "What Faberge often did for classic advertising slogan" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what faberge often did for classic advertising slogan = go to work on an egg.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.