The most common and most likely answer to this crossword clue is the 9 letter word IMPATIENT
I've seen this crossword clue in The Guardian
I
M
P
A
T
I
E
N
T
More clues leading to the result IMPATIENT
- Anxious, I'm one receiving medical care
- Brat, tiny creature seizing man’s neckwear, wanting instant attention?
- I’m customer for a doctor that is restless
- Snappy declaration by doctor's client?
- Intimate shot features political leader becoming restless
- Irritated, Mudd is over case
- Fretful introduction by Irishman perhaps, one on hospital department
- Keen to get on up the motorway to invalid
- Restless, as one waiting for doctor might describe himself
- Writer’s clear about one that’s dying
Buddy explains!

The straight part
We know that "Keen to get on" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case keen to get on = impatient.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 IMPATIENT mean?
IMPATIENT adjective- Restless and intolerant of delays.
- Anxious and eager, especially to begin something.
- Not to be borne; unendurable.
- Prompted by, or exhibiting, impatience.