The most common and most likely answer to this crossword clue is the 2 plus 3 plus 4 letter word ON TOP FORM
I've seen this crossword clue in The Guardian
O
N
T
O
P
F
O
R
M
More clues leading to the result ON TOP FORM
Buddy explains!

The straight part
We know that "doing very well" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case doing very well = on top form.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.