What does the cryptic crossword clue "Free oil on tap is not compulsory (8)" mean?

The definition is "Free". The wordplay uses an anagram. anagram of OIL + ON + TAP = OPTIONAL; definition: "Free" The answer is OPTIONAL.

Free oil on tap is not compulsory (8)

Independent Cryptic #11706 | 17 Across | 2024-04-17 flower