What does the cryptic crossword clue "Gaining knowledge of German? It's complicated (9)" mean?

The definition is "Gaining". The wordplay uses an anagram. anagram of GERMAN? + IT'S = MASTERING; definition: "Gaining" The answer is MASTERING.

Gaining knowledge of German? It's complicated (9)

Daily Mail Cryptic #16959 | 15 Down | 2022-11-02 flower