Section housing a mostly enthusiastic flier (8)
I believe the answer is:
parakeet
'flier' is the definition.
(parakeet is a kind of birdanimal)
'section housing a mostly enthusiastic' is the wordplay.
'section' becomes 'part' ('part' can be a synonym of 'section').
'housing' is an insertion indicator.
'a mostly enthusiastic' becomes 'akee' (I am not sure about this - if you are sure you should believe this answer much more).
'part' placed around 'akee' is 'PARAKEET'.
Can you help me to learn more?
(Other definitions for parakeet that I've seen before include "Small long-tailed bird" , "Brightly-coloured African bird" , "Tropical bird" , "Bird popular as a pet" , "Take pear (anag) - bird" .)