The most common and most likely answer to this crossword clue is the 8 letter word FLYSHEET
I've seen this crossword clue in The Daily Telegraph
F
L
Y
S
H
E
E
T
Buddy explains!

The straight part
We know that "It’s used for canvassing/canvassing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s used for canvassing/canvassing = flysheet.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 FLYSHEET mean?
FLYSHEET noun- An extra cover of a tent to protect against insects and bad weather.
- A printed leaflet or pamphlet.