The most common and most likely answer to this crossword clue is the 8 plus 3 letter word MESOZOIC [?]
I've seen this crossword clue in The Guardian
M
E
S
O
Z
O
I
C
[
?
]
Buddy explains!
The straight part
We know that "from a long time ago" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case from a long time ago = mesozoic [?].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.