The most common and most likely answer to this crossword clue is the 11 plus 2 plus 5 plus 5 letter word OUTSTANDING IN THEIR FIELD
I've seen this crossword clue in The Guardian
O
U
T
S
T
A
N
D
I
N
G
I
N
T
H
E
I
R
F
I
E
L
D
Buddy explains!
The straight part
We know that "doing a great job" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case doing a great job = outstanding in their field.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.