The most common and most likely answer to this crossword clue is the 6 letter word ACTANT
A
C
T
A
N
T
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The straight part
We know that "one functions as an agent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one functions as an agent = actant.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 ACTANT mean?
ACTANT noun- (grammar) Any of the participants, such as the subject or object, in a grammatical clause.