The most common and most likely answer to this crossword clue is the 9 letter word CAPUCHINS
I've seen this crossword clue in The Guardian
C
A
P
U
C
H
I
N
S
More clues leading to the result CAPUCHINS
Buddy explains!

The straight part
We know that "Monkeys" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case monkeys = capuchins.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 CAPUCHINS mean?
CAPUCHINS noun- A monk in the Order of Friars Minor Capuchin; (generally) a Franciscan.
- A garment consisting of a cloak and hood, made in imitation of the dress of Capuchin monks.
- A capuchin monkey.
- A hooded pigeon.