The most common and most likely answer to this crossword clue is the 10 letter word PROCESSION
I've seen this crossword clue in The Daily Telegraph
P
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Buddy explains!

The straight part
We know that "file" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case file = procession.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 PROCESSION mean?
PROCESSION noun- The act of progressing or proceeding.
- A group of people or things moving along in an orderly, stately, or solemn manner; a train of persons advancing in order; a retinue.
- A number of things happening in sequence (in space or in time).
- (in the plural) Litanies said in procession and not kneeling.
- To take part in a procession.
- To honour with a procession.
- (North Carolina and Tennessee) To ascertain, mark, and establish the boundary lines of (lands).