The most common and most likely answer to this crossword clue is the 2 plus 7 letter word ON MESSAGE
O
N
M
E
S
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A
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Buddy explains!
The straight part
We know that "sharing party line" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sharing party line = on message.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 ON MESSAGE mean?
ON MESSAGE adjective- Publicly reiterating and supporting the official party policy; conforming to the party’s agreed policies.