The most common and most likely answer to this crossword clue is the 5 plus 5 plus 3 letter word KNOCK SPOTS OFF
I've seen this crossword clue in The Times
K
N
O
C
K
S
P
O
T
S
O
F
F
More clues leading to the result KNOCK SPOTS OFF
Buddy explains!
The straight part
We know that "getting better by miles" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case getting better by miles = knock spots 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.