The most common and most likely answer to this crossword clue is the 9 letter word SPIKENARD
I've seen this crossword clue in The Guardian
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More clues leading to the result SPIKENARD
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The straight part
We know that "Essential oil" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case essential oil = spikenard.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 SPIKENARD mean?
SPIKENARD noun- A perfumed ointment, extracted from the plant Nardostachys jatamansi that belongs to the Valerian family and grows in the Himalayas.
- The plant Nardostachys jatamansi (syn. Nardostachys grandiflora).
- Lavandula stoechas, another species used in antiquity to produce an aromatic oil.
- Any plant of the genus Aralia of the Araliaceae family.
- False spikenard (Maianthemum racemosum, syn. Smilacina racemosa)
- Ploughman's spikenard (Inula conyza, now Pentanema conyzae).
- Wild spikenard (Asarum europaeum).