The most common and most likely answer to this crossword clue is the 5 plus 2 plus 3 plus 5 plus 6 letter word THICK AS TWO SHORT PLANKS
I've seen this crossword clue in Independent
T
H
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C
K
A
S
T
W
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S
H
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R
T
P
L
A
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K
S
Buddy explains!
The straight part
We know that "Relative density expressed as 2x2x4?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case relative density expressed as 2x2x4? = thick as two short planks.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.