The most common and most likely answer to this crossword clue is the 5 plus 4 letter word FIELD TEST
I've seen this crossword clue in The Guardian
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More clues leading to the result FIELD TEST
Buddy explains!

The straight part
We know that "experiment in practice" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case experiment in practice = field test.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 FIELD TEST mean?
FIELD TEST verb- To test something using the conditions that it was designed to operate under, especially out in the real world instead of in a laboratory or workshop.