The most common and most likely answer to this crossword clue is the 9 letter word ACROPOLIS
I've seen this crossword clue in Independent
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More clues leading to the result ACROPOLIS
- What might be rice and grain store up in citadel
- Car pool is relocated in classical site
- Cop and sailor fighting for citadel
- A gathering of Labour is high point for city
- Paris cool about attraction of Athens
- A group put up storage tower in the citadel
- King with work in store around westward citadel
- Citadel produces a yield on bad soil
- Large old building: a place for holding grain erected to hold grain?
- Cool Paris trashed Athenian citadel
Buddy explains!

The straight part
We know that "Large old building" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case large old building = acropolis.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 ACROPOLIS mean?
ACROPOLIS noun- A promontory (usually fortified with a citadel) forming the hub of many Grecian cities, and around which many were built for defensive purposes before and during the classical period; compare Acropolis.