The most common and most likely answer to this crossword clue is the 4 letter word MUGS
M
U
G
S
Buddy explains!

The straight part
We know that "Faces/simpletons" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case faces/simpletons = mugs.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 MUGS mean?
MUGS noun- A large cup for hot liquids, usually having a handle and used without a saucer.
- The face, often used deprecatingly.
- A gullible or easily-cheated person.
- A stupid or contemptible person.
- To strike in the face.
- To assault for the purpose of robbery.
- To exaggerate a facial expression for communicative emphasis; to make a face, to pose, as for photographs or in a performance, in an exaggerated or affected manner.
- To photograph for identification; to take a mug shot.
- To learn or review a subject as much as possible in a short time; cram.
- Motherfucker (usually in similes, e.g. "like a mug" or "as a mug")