The most common and most likely answer to this crossword clue is the 8 letter word SQUEEGEE
I've seen this crossword clue in The Guardian
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More clues leading to the result SQUEEGEE
Buddy explains!

The straight part
We know that "a window-cleaner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a window-cleaner = squeegee.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 SQUEEGEE mean?
SQUEEGEE noun- A tool consisting of a rubber or similar blade attached at a right angle to a handle, particularly
- A roller used to similar effect, particularly
- A person who uses a squeegee, especially one who "cleans" the windshield of a car stopped at a traffic light and then demands payment.
- To use a squeegee.