The most common and most likely answer to this crossword clue is the 9 letter word RECTIFIES
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Buddy explains!

The straight part
We know that "mends" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mends = rectifies.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 RECTIFIES mean?
RECTIFIES verb- To heal (an organ or part of the body).
- To restore (someone or something) to its proper condition; to straighten out, to set right.
- To remedy or fix (an undesirable state of affairs, situation etc.).
- To purify or refine (a substance) by distillation.
- To correct or amend (a mistake, defect etc.).
- To correct (someone who is mistaken).
- (geodesy) To adjust (a globe or sundial) to prepare for the solution of a proposed problem.
- To convert (alternating current) into direct current.
- To determine the length of a curve included between two limits.
- To produce (as factitious gin or brandy) by redistilling bad wines or strong spirits (whisky, rum, etc.) with flavourings.