The most common and most likely answer to this crossword clue is the 3 plus 6 plus 2 plus 3 plus 4 letter word THE CHERRY ON THE CAKE
I've seen this crossword clue in Financial Times
T
H
E
C
H
E
R
R
Y
O
N
T
H
E
C
A
K
E
Buddy explains!
The straight part
We know that "topper" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case topper = the cherry on the cake.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.