Cryptic crossword clue
Very old, bright and roomy 10

The most common and most likely answer to this crossword clue is the 10 letter word VOLUMINOUS

I've seen this crossword clue in The Guardian
V
O
L
U
M
I
N
O
U
S

Buddy explains!

The straight part
We know that "roomy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case roomy = voluminous.
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 VOLUMINOUS mean?

VOLUMINOUS adjective
  1. Of or pertaining to volume or volumes.
  2. Consisting of many folds, coils, or convolutions.
  3. Of great volume, or bulk; large.
  4. Having written much, or produced many volumes

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