The most common and most likely answer to this crossword clue is the 12 letter word WALK-THROUGH
I've seen this crossword clue in The Guardian
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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 WALK-THROUGH mean?
WALK-THROUGH adjective- That can be walked through
- Of passenger carriages in a train, having no doors in gangway connections between the carriages, creating a "see-through" effect through the train.
- The process of inspecting algorithms and source code by following paths through the algorithms or code as determined by input conditions and choices made along the way.
- A playthrough that details the steps involved in winning the game.
- A financial audit that traces a sample transaction through the system to ensure that it is processed and reported correctly.
- A theatrical or film rehearsal in which the actors move around the stage or set but are not in costume.