The most common and most likely answer to this crossword clue is the 6 letter word BILLED
I've seen this crossword clue in The Daily Telegraph
B
I
L
L
E
D
Buddy explains!
The straight part
We know that "supposed to be taking part" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case supposed to be taking part = billed.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 BILLED mean?
BILLED verb- To dig, chop, etc., with a bill.
- To peck
- To stroke bill against bill, with reference to doves; to caress in fondness
- To advertise by a bill or public notice.
- To charge; to send a bill to.
- Having a specified kind of bill (beak or beak-like projection).