The most common and most likely answer to this crossword clue is the 12 letter word FUNDAMENTALS
I've seen this crossword clue in The Guardian
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More clues leading to the result FUNDAMENTALS
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.
Crossword Answer definitions
FUNDAMENTALS noun- (usually in the plural) A leading or primary principle, rule, law, or article, which serves as the groundwork of a system; an essential part
- The lowest frequency of a periodic waveform.
- The lowest partial of a complex tone.
- The fundamental analysis data.