The most common and most likely answer to this crossword clue is the 9 letter word ACTIVATOR
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Buddy explains!
The straight part
We know that "agent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case agent = activator.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 ACTIVATOR mean?
ACTIVATOR noun- One who, or that which, activates.
- Something that activates a catalyst
- Any chemical or agent which regulates one or more genes by increasing the rate of transcription.
- A type of effector that increases the rate of enzyme-mediated reactions.
- A type of dopant used in phosphors and scintillators.
- An agent that enables the flotation of a mineral or minerals.