Bird's nest is one being very high (4)
I believe the answer is:
soup
'bird's nest is one' is the definition.
(bird's nest soup is a type of soup)
'very high' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'high' becomes 'up' (associated in meaning).
'so'+'up'='SOUP'
'being' acts as a link.
(Other definitions for soup that I've seen before include "Liquid dish" , "Cock-a-leekie, for example" , "Opus (anag) - typical starter" , "Liquid - course" , "Bisque, for example" .)