Advertisement for doctor (5)
I believe the answer is:
promo
'advertisement' is the definition.
(short for promotion)
'for doctor' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'doctor' becomes 'mo' (medical officer).
'pro'+'mo'='PROMO'
(Other definitions for promo that I've seen before include "advertisement" , "Advertising film or video" , "publicity event" , "for [a] little while" , "Publicity for project or product (abbr)" .)