The most common and most likely answer to this crossword clue is the 8 letter word DEGRADED
I've seen this crossword clue in The Daily Telegraph
D
E
G
R
A
D
E
D
More clues leading to the result DEGRADED
Buddy explains!

The straight part
We know that "Worn down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case worn down = degraded.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 DEGRADED mean?
DEGRADED verb- To lower in value or social position.
- To reduce in quality or purity.
- To reduce in altitude or magnitude, as hills and mountains; to wear down.
- Feeling or having undergone degradation; deprived of dignity or self-respect.
- Having the typical characters or organs in a partially developed condition, or lacking certain parts.
- Having steps; said of a cross whose extremities end in steps growing larger as they leave the centre; on degrees.