The most common and most likely answer to this crossword clue is the 10 letter word CALUMNIATE
I've seen this crossword clue in The Daily Telegraph
C
A
L
U
M
N
I
A
T
E
More clues leading to the result CALUMNIATE
Buddy explains!
The straight part
We know that "smear" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case smear = calumniate.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.
What does CALUMNIATE mean?
CALUMNIATE verb- To make hurtful untrue comments about.
- To levy a false charge against, especially of a vague offense, with the intent to damage someone's reputation or standing.