So-called 'bird' initially uncle labelled as 'reptile (7)
I believe the answer is:
titular
'so-called' is the definition.
(thesaurus)
'bird initially uncle labelled as reptile' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'initially' says to take the initial letters.
The initial letters of 'uncle labelled as reptile' is 'ular'.
'tit'+'ular'='TITULAR'
(Other definitions for titular that I've seen before include "See" , "Having name of ruler but without real authority" , "So called" , "Relating to title" , "Honorary" .)