The most common and most likely answer to this crossword clue is the 10 letter word STIGMATISE
I've seen this crossword clue in The Daily Telegraph
S
T
I
G
M
A
T
I
S
E
More clues leading to the result STIGMATISE
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 = stigmatise.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 STIGMATISE mean?
STIGMATISE verb- To characterize as disgraceful or ignominious; to mark with a stigma or stigmata.