The most common and most likely answer to this crossword clue is the 7 letter word OZONISE
I've seen this crossword clue in The Times
O
Z
O
N
I
S
E
Buddy explains!
The straight part
We know that "Treat with powerful agent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case treat with powerful agent = ozonise.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 OZONISE mean?
OZONISE verb- To treat or react with ozone; to ozonate
- To convert oxygen into ozone, especially by using an ozonizer