The most common and most likely answer to this crossword clue is the 10 letter word FOSSILISED
F
O
S
S
I
L
I
S
E
D
More clues leading to the result FOSSILISED
Buddy explains!

The straight part
We know that "Out of date" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case out of date = fossilised.The cryptic/wordplay part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 FOSSILISED mean?
FOSSILISED adjective- In a state of fossilization; preserved in rock
- Outmoded
- Having become a fossil: no longer productive