The most common and most likely answer to this crossword clue is the 9 letter word SPONGEBOB
I've seen this crossword clue in Independent
S
P
O
N
G
E
B
O
B
More clues leading to the result SPONGEBOB
Buddy explains!
The straight part
We know that "animated character who’s lost his trousers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case animated character who’s lost his trousers = spongebob.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.