The most common and most likely answer to this crossword clue is the 3 plus 3 letter word TOP CAT
I've seen this crossword clue in The Guardian
T
O
P
C
A
T
More clues leading to the result TOP CAT
Buddy explains!

The straight part
We know that "Cartoon series/f/i" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cartoon series/f/i = top cat.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.