The most common and most likely answer to this crossword clue is the 12 letter word INTERVENTION
I've seen this crossword clue in The Times
I
N
T
E
R
V
E
N
T
I
O
N
Buddy explains!
The straight part
We know that "agency" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case agency = intervention.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 INTERVENTION mean?
INTERVENTION noun- The action of intervening; interfering in some course of events.
- A legal motion through which a person or entity who has not been named as a party to a case seeks to have the court order that they be made a party.
- An orchestrated attempt to convince somebody with an addiction or other psychological problem to seek professional help and/or change their behavior.
- An action taken or procedure performed; an operation.