Population which has not been studied extensively by sociologists, and other researches. What is the source? It could mean not studied extensively for the purpose of a particular study, compared to other groups.
It means that not much sociological research has been done on adolescents in minority groups (Muslims in the United Kingdom, for example). It's incorrect English. If anyone said it was correct English, I'd say it was sloppy at best.
You could say 'Little research has been done on adolescents in minority groups.' An understudy in the theatre is someone who deputises for an actor/actress when he/she falls ill.