The most common and most likely answer to this crossword clue is the 6 letter word SCALED
I've seen this crossword clue in The Sunday Telegraph
S
C
A
L
E
D
Buddy explains!

The straight part
We know that "Mounted" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mounted = scaled.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 SCALED mean?
SCALED verb- To change the size of something whilst maintaining proportion; especially to change a process in order to produce much larger amounts of the final product.
- To climb to the top of.
- To tolerate significant increases in throughput or other potentially limiting factors.
- To weigh, measure or grade according to a scale or system.
- To remove the scales of.
- To become scaly; to produce or develop scales.
- To strip or clear of scale; to descale.
- To take off in thin layers or scales, as tartar from the teeth; to pare off, as a surface.
- To separate and come off in thin layers or laminae.
- To scatter; to spread.
- To clean, as the inside of a cannon, by the explosion of a small quantity of powder.
- Covered with scales or scale-like structures.
- Without scales, or with the scales removed.