Cherished desire? A doctor has it, one needing to get on (8)
I believe the answer is:
Here is my best explanation:
'cherished desire?' is the definition.
'a doctor has it one needing to get on' is the subsidiary indication.
'a doctor has' becomes 'ambi' (I can't justify this - if you can you should believe this answer much more).
'it' becomes 't' (abbreviation. e.g. in 'tis).
'one' becomes 'i' (Roman numeral).
'needing to get' means one lot of letters go next to another.
(Other definitions for ambition that I've seen before include "Desire to succeed in the world", "Desire to achieve success", "Cherished aspiration", "Desire (for riches, fame)", "Strong desire for success", "Desire for power, fame, etc", "Desire to succeed in life", "Desire for success or power", "Urge to better oneself" and "A determination to succeed".)