The most common and most likely answer to this crossword clue is the 9 letter word SHEEP-DIP
I've seen this crossword clue in Independent
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Buddy explains!
The straight part
We know that "Disinfectant that will benefit you orally" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case disinfectant that will benefit you orally = sheep-dip.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 SHEEP-DIP mean?
SHEEP-DIP noun- A liquid formulation of insecticide and fungicide, used to protect sheep from infestation by external parasites.
- To replace someone's public service in the military and/or intelligence with another pretended occupation in order for them to serve as a covert intelligence agent.