The most common and most likely answer to this crossword clue is the 12 letter word CHEMOTHERAPY
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The straight part
We know that "treatment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case treatment = chemotherapy.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 CHEMOTHERAPY mean?
CHEMOTHERAPY noun- Any chemical treatment intended to be therapeutic with respect to a disease state.
- (most common usage) chemical treatment to kill or halt the replication and/or spread of cancerous cells in a patient.