The most common and most likely answer to this crossword clue is the 10 letter word THUNDERBOX
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "Hollow trough beneath raised seat forming /Hollow trough beneath raised seat forming /primitive loo" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hollow trough beneath raised seat forming /hollow trough beneath raised seat forming /primitive loo = thunderbox.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 THUNDERBOX mean?
THUNDERBOX noun- A close-stool, a stool enclosing a chamber pot.
- An outhouse or latrine: a rudimentary outdoor toilet.
- A box of metal balls used to create a thunder sound effect.