The most common and most likely answer to this crossword clue is the 8 letter word MANDRILL
M
A
N
D
R
I
L
L
More clues leading to the result MANDRILL
- Primate's staff ailing — doctor called in
- Fellow has to train monkey
- School staff first to find baboon
- Habits of the male baboon
- Fellow bit monkey
- Ape soldier on military exercise
- Staff physician sick – his bottom is blue!
- Primate, in practice, preceded by bishop, say
- Intelligent primate training another
- Limitations shown by master with sick animal
Buddy explains!

The straight part
We know that "his bottom is blue" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case his bottom is blue = mandrill.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 MANDRILL mean?
MANDRILL noun- A primate, Mandrillus sphinx, with colorful face and rump.