Robert Oppenheimer's first association with a songbird (8)
I believe the answer is:
bobolink
'a songbird' is the definition.
'robert oppenheimer's first association' is the wordplay.
'robert' becomes 'bob' (I've seen this before).
'first' suggests taking the first letters.
'association' becomes 'link'.
The first letter of 'oppenheimers' is 'o'.
'bob'+'o'+'link'='BOBOLINK'
'with' is the link.
(Other definitions for bobolink that I've seen before include "N. American songbird" , "American bird" , "Ricebird" .)