So rich doctor formed singing groups (6)
I believe the answer is:
choirs
'singing groups' is the definition.
(I've seen this before)
'so rich doctor' is the wordplay.
'doctor' indicates anagramming the letters.
'so'+'rich'='sorich'
'sorich' anagrammed gives 'CHOIRS'.
'formed' acts as a link.
(Other definitions for choirs that I've seen before include "Groups of singers, in church say" , "Singing groups" , "Groups of singers, often in churches" , "Singing teams" , "Bands of singers" .)