The most common and most likely answer to this crossword clue is the 9 letter word FORMALISM
I've seen this crossword clue in The Guardian
F
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Buddy explains!
The straight part
We know that "Purely structural approach" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case purely structural approach = formalism.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 FORMALISM mean?
FORMALISM noun- Strict adherence to a given form of conduct, practice etc.
- One of several alternative computational paradigms for a given theory.
- An approach to interpretation and/or evaluation focused on the (usually linguistic) structure of a literary work rather than on the contexts of its origin or reception.
- The tendency to elevate formal above expressive value in music, as in serialism.
- A particular mathematical or scientific theory or description of a given state or effect.
- A formal expression of a grammar; a formal grammar; a set of rules of syntax that, without reference to semantics, determine whether a sequence of symbols is a well-formed sentence in a given formal language.