The most common and most likely answer to this crossword clue is the 12 letter word STAKEHOLDERS
S
T
A
K
E
H
O
L
D
E
R
S
Buddy explains!

The straight part
We know that "Interested parties" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case interested parties = stakeholders.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 STAKEHOLDERS mean?
STAKEHOLDERS noun- A person holding the stakes of bettors, with the responsibility of delivering the pot to the winner of the bet.
- An escrow agent or custodian.
- A person filing an interpleader action, such as a garnishee or trustee, who acknowledges possession of property that is owed to one or more of several other claimants.
- A person or organisation with a legitimate interest in a given situation, action or enterprise.