You and I discovered bird act is eccentric (6)
I believe the answer is:
weirdo
'is eccentric' is the definition.
(I've seen this before)
'you and i discovered bird act' is the wordplay.
'you and i' becomes 'we'.
'discovered' says to take the centre (dis-covered - outer letters removed).
'act' becomes 'do' (to do is to act or execute a task).
The middle of 'bird' is 'ir'.
'we'+'ir'+'do'='WEIRDO'
(Other definitions for weirdo that I've seen before include "Informally, an odd or eccentric person" , "Odd type" , "Odd bod" , "queer fish" , "Way-out person" .)