The most common and most likely answer to this crossword clue is the 9 letter word DOORNAILS
I've seen this crossword clue in The Guardian
D
O
O
R
N
A
I
L
S
Buddy explains!
The straight part
We know that "one of these is as dead as a dodo" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one of these is as dead as a dodo = doornails.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 DOORNAILS mean?
DOORNAILS noun- A nail with a wide head, traditionally used in the construction and ornamentation of wooden doors.