The most common and most likely answer to this crossword clue is the 11 letter word ADULTERATED
I've seen this crossword clue in The Guardian
A
D
U
L
T
E
R
A
T
E
D
More clues leading to the result ADULTERATED
Buddy explains!

The straight part
We know that "devalued" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case devalued = adulterated.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 ADULTERATED mean?
ADULTERATED verb- To corrupt.
- To spoil by adding impurities.
- To commit adultery.
- To defile by adultery.
- Mixed with impurities