The most common and most likely answer to this crossword clue is the 12 letter word LEXICOGRAPHY
I've seen this crossword clue in The Guardian
L
E
X
I
C
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G
R
A
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Buddy explains!

The straight part
We know that "Johnson’s work" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case johnson’s work = lexicography.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 LEXICOGRAPHY mean?
LEXICOGRAPHY noun- The art or craft of compiling, writing, and editing dictionaries.
- The scholarly discipline of analysing and describing the semantic, syntagmatic and paradigmatic relationships within the lexicon (vocabulary) of a language and developing theories of dictionary components and structures linking the data in dictionaries.
- A dictionary, a lexicon, a wordbook.