This is very easy ... gee, the setter's Bluebird! (5)
I believe the answer is:
gimme
'this is very easy' is the definition.
The definition suggests a singular noun but the answer is not.
'gee the setter's bluebird' is the wordplay.
'gee' becomes 'g' (synonyms).
'the setter's' becomes 'Im' (referring to the crossword setter).
'bluebird' becomes 'me' (I can't justify this - if you can you should believe this answer much more).
'g'+'im'+'me'='GIMME'
Can you help me to learn more?
(Other definitions for gimme that I've seen before include "sitter" , "I rudely demand" , "Hand (it) to me (informal)" , "Reason for unnecessarily putting" , "Green concession" .)