The most common and most likely answer to this crossword clue is the 11 letter word DISTRIBUTOR
I've seen this crossword clue in The Daily Telegraph
D
I
S
T
R
I
B
U
T
O
R
More clues leading to the result DISTRIBUTOR
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 = distributor.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 DISTRIBUTOR mean?
DISTRIBUTOR noun- One who or that which distributes.
- A device that distributes electric current, especially to the spark plugs in an internal combustion engine.
- A machine for distributing type.
- An appliance, such as a roller, in a printing press, for distributing ink.