The most common and most likely answer to this crossword clue is the 9 letter word INOCULATE
I've seen this crossword clue in The Times
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More clues leading to the result INOCULATE
- Inject copper into deformed toenail
- To guard against infection I clean out potty
- Jab trendy old clergyman when lecturer replaces rector
- Graft in turning around company — advanced without union
- Protect against wild speculation
- Inject iodine, then clean out wound
- Lunatic fringes of Oxbridge cast ready for travel abroad?
- Imbue a clue into puzzle
- Independent having a clue not cracked? That calls for graft
- To guard against infection I clean out wound
Buddy explains!

The straight part
We know that "Jab" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jab = inoculate.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 INOCULATE mean?
INOCULATE verb- To introduce an antigenic substance or vaccine into something (e.g. the body) or someone, such as to produce immunity to a specific disease.
- (by extension) To safeguard or protect something as if by inoculation.
- To add one substance to another; to spike.
- To graft by inserting buds.
- To introduce into the mind (used especially of harmful ideas or principles); to imbue; to implant.