The most common and most likely answer to this crossword clue is the 5 plus 3 letter word THROW OFF
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More clues leading to the result THROW OFF
Buddy explains!

The straight part
We know that "Junk" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case junk = throw off.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 THROW OFF mean?
THROW OFF noun- A start in a hunt or a race.
- A throw taken from the centre line which resumes play after each goal and at the start of each period.