Endlessly ardent enthusiast, one hugging posh birds (8)
I believe the answer is:
avifauna
'birds' is the definition.
(term for the birds of a particular region)
'endlessly ardent enthusiast one hugging posh' is the wordplay.
'endlessly ardent' becomes 'avi' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'enthusiast' becomes 'fan' (fan is a kind of enthusiast).
'one' becomes 'a'.
'hugging' indicates putting letters inside.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'fan'+'a'='fana'
'fana' enclosing 'u' is 'fauna'.
'avi'+'fauna'='AVIFAUNA'
(Other definitions for avifauna that I've seen before include "The birds of a particular region" , "Local birds" , "Bird collection" , "Flyers locally" .)