Doctor courted a mistress, say (8)
I believe the answer is:
educator
'mistress say' is the definition.
(mistress is a kind of educator)
'doctor courted a' is the wordplay.
'doctor' indicates anagramming the letters.
'courted'+'a'='courteda'
'courteda' anagrammed gives 'EDUCATOR'.
(Other definitions for educator that I've seen before include "Pedagogue, teacher" , "Teacher, lecturer" , "Trainer, teacher" , "Instructor" , "mistress, say" .)