Cure? Doctor runs to source of medicine (7)
I believe the answer is:
nostrum
'cure?' is the definition.
(nostrum is a kind of cure)
'doctor runs to source of medicine' is the wordplay.
'doctor' indicates anagramming the letters.
'source of' indicates taking the first letters.
The initial letter of 'medicine' is 'm'.
'runs'+'to'='runsto'
'runsto' is an anagram of 'nostru'.
'nostru'+'m'='NOSTRUM'
(Other definitions for nostrum that I've seen before include "Tonic" , "our Latin" , "Quack cure" , "Quack medicine - run most (anag)" , "Hypothetical remedy for all ills" .)