The happy medium may result from a successful one (6)
I believe the answer is:
seance
'the happy medium may' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'from a successful one' is the wordplay.
I cannot quite see how this works, but
'from' could be 'e' (I've seen this in other clues) and 'e' is found within the answer.
'a' could be 'an' and 'an' is found in the answer.
This explanation may well be incorrect...
'result' is the link.
Can you help me to learn more?
(Other definitions for seance that I've seen before include "Spiritualist sitting" , "Meeting with spiritualist present" , "Medium's session" , "Spiritualist session with medium in charge" , "Spiritualist meeting presided over by medium" .)