The most common and most likely answer to this crossword clue is the 5 plus 5 letter word ORDER PAPER
I've seen this crossword clue in Independent
O
R
D
E
R
P
A
P
E
R
More clues leading to the result ORDER PAPER
Buddy explains!
The straight part
We know that "source of parliamentary information" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case source of parliamentary information = order paper.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.