The most common and most likely answer to this crossword clue is the 15 letter word INSTRUMENTALITY
I've seen this crossword clue in The Times
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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 = instrumentality.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 INSTRUMENTALITY mean?
INSTRUMENTALITY noun- The quality or condition of being instrumental; serving a purpose, being useful.
- A governmental organ with a specific purpose.
- Something that is instrumental; an instrument.