The most common and most likely answer to this crossword clue is the 6 letter word RESOLE
I've seen this crossword clue in The Daily Telegraph
R
E
S
O
L
E
Buddy explains!

The straight part
We know that "repair at last" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case repair at last = resole.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 RESOLE mean?
RESOLE verb- To replace or reattach the sole of an article of footwear.