Nosy person in active majority (8)
I believe the answer is:
busybody
'nosy' is the definition.
(thesaurus)
'person in active majority' is the wordplay.
'person' becomes 'bod' (bod can informally mean a person or body).
'in' indicates putting letters inside.
'active' becomes 'busy' (similar in meaning).
'majority' becomes 'y' (I can't justify this - if you can you should believe this answer much more).
'busy'+'y'='busyy'
'bod' inserted into 'busyy' is 'BUSYBODY'.
Can you help me to learn more?
(Other definitions for busybody that I've seen before include "he'll nose around" , "Prying person" , "One meddling" , "Meddling person" , "A meddlesome person, certainly not idle" .)