The most common and most likely answer to this crossword clue is the 12 letter word INTERSTITIAL
I've seen this crossword clue in Independent
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Buddy explains!

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 INTERSTITIAL mean?
INTERSTITIAL noun- A webpage, usually carrying advertising, displayed when leaving one content page for another.
- An interstitial discontinuity in a crystal.
- Of, relating to, or situated in an interstice.