Young creature identified by US city doctor (4)
I believe the answer is:
lamb
'young creature' is the definition.
(I've seen this before)
'us city doctor' is the wordplay.
'us city' becomes 'LA' (Los Angeles).
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'la'+'mb'='LAMB'
'identified by' acts as a link.
(Other definitions for lamb that I've seen before include "Writer" , "Mary, and Charles?" , "Charles ---, essayist; meek animal" , "Noted essayist - symbol of innocence" , "essayists?" .)