Bird but not birdie (5)
I believe the answer is:
eagle
This clue might be a double definition.
'bird but' is the first definition.
'eagle' can be an answer for 'bird' (eagle is a type of bird). I'm unsure of the 'but' bit.
'not birdie' is the second definition.
'eagle' can be an answer for 'birdie' (thesaurus). I'm not sure about the 'not' bit.
Can you help me to learn more?
(Other definitions for eagle that I've seen before include "Bird of prey; former children's comic" , "Score of two strokes under par for a golf hole" , "Large birds of prey." , "Two under in 20 across" , "experts are putting more than albatross?" .)