The most common and most likely answer to this crossword clue is the 9 letter word POISONING
I've seen this crossword clue in The Guardian
P
O
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S
O
N
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Buddy explains!

The straight part
We know that "Infecting" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case infecting = poisoning.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 POISONING mean?
POISONING verb- To use poison to kill or paralyse (somebody).
- To pollute; to cause to become poisonous.
- To cause to become much worse.
- To cause (someone) to hate or to have unfair negative opinions.
- To inhibit the catalytic activity of.
- To place false information into (a cache) as part of an exploit.
- The administration of a poison.
- The state of being poisoned.