Doctor, I have ambition! (5)
I believe the answer is:
drive
'ambition' is the definition.
(I know that ambition can be written as drive)
'doctor i have' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'i have' becomes 'ive' (contraction of 'I have').
'dr'+'ive'='DRIVE'
(Other definitions for drive that I've seen before include "run saloon?" , "Force into action, impel" , "Impel; house approach" , "Motivate" , "sustained military assault" .)