Third letter about Bumpkin Bill's bird (9)
I believe the answer is:
chickadee
'bird' is the definition.
(chickadee is a kind of bird)
'third letter about bumpkin bill's' is the wordplay.
'third letter' becomes 'cee' (I've seen this in another clue).
'about' is an insertion indicator.
'bumpkin' becomes 'hick' (synonyms).
'bill' becomes 'ad' (advertisement stuck on wall).
'hick'+'ad'='hickad'
'cee' enclosing 'hickad' is 'CHICKADEE'.
(Other definitions for chickadee that I've seen before include "flier from America" , "US bird" .)