What does the cryptic crossword clue "Old unit I suspect is weakening (8)" mean?

The definition is "weakening". The wordplay uses an anagram. anagram of OLD + UNIT + I = DILUTION; definition: "weakening" The answer is DILUTION.

Old unit I suspect is weakening (8)

Daily Mail Cryptic #17293 | 1 Across | 2024-02-14 flower