The most common and most likely answer to this crossword clue is the 8 letter word UNVALUED
I've seen this crossword clue in The Guardian
U
N
V
A
L
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D
Buddy explains!
The straight part
We know that "considered worthless" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case considered worthless = unvalued.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 UNVALUED mean?
UNVALUED adjective- Not having been valued or appraised.
- Not considered to be of worth; deemed valueless.
- Having inestimable value; invaluable.