The most common and most likely answer to this crossword clue is the 13 letter word PANTECHNICONS
I've seen this crossword clue in The Times
P
A
N
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Buddy explains!

The straight part
We know that "big vans" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case big vans = pantechnicons.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 PANTECHNICONS mean?
PANTECHNICONS noun- A building or place housing shops or stalls where all sorts of (especially exotic) manufactured articles are collected for sale.
- Originally pantechnicon van: a van, especially a large moving or removal van.