Note from yours truly to doctor (4)
I believe the answer is:
memo
'note' is the definition.
(a memo is a short note)
'yours truly to doctor' is the wordplay.
'yours truly' becomes 'me' (the person who set this crossword).
'to doctor' becomes 'mo' (medical officer).
'me'+'mo'='MEMO'
'from' is the link.
(Other definitions for memo that I've seen before include "Written business message, in short" , "Short note or message" , "A little something to be remembered" , "- Currie, Br. politician" , "It will remind" .)