The most common and most likely answer to this crossword clue is the 9 letter word DISTEMPER
I've seen this crossword clue in Financial Times
D
I
S
T
E
M
P
E
R
More clues leading to the result DISTEMPER
- Paint that makes dog sick
- Boxers may get sick of this underworld character
- Little girl's fit of anger that may affect dog
- Paint Sid knocked over with outburst of anger
- Before tantrum harshly criticise painting technique
- Paint a sickener for animal
- Underworld character seen in painting
- Underworld god to mitigate best friend’s condition
- Paint is hell to soften
- Paint a hunk of a nudist emperor
Buddy explains!

The straight part
We know that "best friend’s condition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case best friend’s condition = distemper.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 DISTEMPER mean?
DISTEMPER noun- A viral disease of animals, such as dogs and cats, characterised by fever, coughing and catarrh.
- A disorder of the humours of the body; a disease.
- A glue-based paint.
- A painting produced with this kind of paint.
- To temper or mix unduly; to make disproportionate; to change the due proportions of.
- To derange the functions of, whether bodily, mental, or spiritual; to disorder; to disease.
- To deprive of temper or moderation; to disturb; to ruffle; to make disaffected, ill-humoured, or malignant.
- To intoxicate.
- To paint using distemper.
- To mix (colours) in the way of distemper.