Extremely happy being in elite classification (6)
I believe the answer is:
phylum
'classification' is the definition.
(thesaurus)
'extremely happy being in elite' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'being in' indicates putting letters inside.
'elite' becomes 'plum' (I am not sure about this - if you are sure you should believe this answer much more).
'happy' with its centre taken out is 'hy'.
'hy' placed into 'plum' is 'PHYLUM'.
Can you help me to learn more?
(Other definitions for phylum that I've seen before include "part of the kingdom" , "botany division" , "Taxonomic category above class" , "Biological group" , "one of several in kingdom?" .)