The most common and most likely answer to this crossword clue is the 8 letter word RELIEVER
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Buddy explains!

The straight part
We know that "Back-up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case back-up = reliever.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 RELIEVER mean?
RELIEVER noun- Something which relieves (pain, etc.).
- A relief pitcher.
- Someone who fills in for another.
- A garment kept for being lent out.