The most common and most likely answer to this crossword clue is the 12 letter word INSTALLATION
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result INSTALLATION
- Apparatus positioned at Paddington, say, letting everyone in
- Hold up in one country military establishment
- Establishment play for time beset by independent people
- One 22 in country establishment
- Island country about to delay military base
- Military establishment at Waterloo, say, with everyone inside
- A bit of life needed in new nationalist art exhibit
- Equipment in office, everything fitting in
- The establishment in place, everyone comes in
- I play for time during race for investiture
Buddy explains!

The straight part
We know that "investiture" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case investiture = installation.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 INSTALLATION mean?
INSTALLATION noun- An act of installing.
- Something installed, especially the whole of a system of machines, apparatus, and accessories, when set up and arranged for practical working, as in electric lighting, transmission of power, etc.
- A work of installation art.
- A grouping of facilities that constitute a permanent military base.