The most common and most likely answer to this crossword clue is the 5 letter word MARES
I've seen this crossword clue in The Guardian
M
A
R
E
S
Buddy explains!
The straight part
We know that "Mounts" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mounts = mares.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 MARES mean?
MARES noun- An adult female horse.
- A foolish woman.
- A type of evil spirit formerly thought to sit on the chest of a sleeping person; also, the feeling of suffocation felt during sleep, attributed to such a spirit.
- A demon or monster, thought to plague people while they slept and cause a feeling of suffocation and terror during sleep.
- Sleep paralysis.
- A very bad or frightening dream.
- Any bad, miserable, difficult or terrifying situation or experience that arouses anxiety, terror, agony or great displeasure.