What does the cryptic crossword clue "After treatment, I retain lethargy (7)" mean?

The definition is "lethargy". The wordplay uses an anagram. anagram of I + RETAIN = INERTIA; definition: "lethargy" The answer is INERTIA.

After treatment, I retain lethargy (7)

Daily Mail Cryptic #17373 | 21 Down | 2024-06-05 flower