Birds flew at last over eastern States (7)
I believe the answer is:
weavers
'birds' is the definition.
(weaver is a kind of bird)
'flew at last over eastern states' is the wordplay.
'at last' says to take the final letters.
'over' says to put letters next to each other (in a down clue, some letters go over others).
'eastern' becomes 'e'.
'states' becomes 'avers' (to aver is to declare or state).
The final letter of 'flew' is 'w'.
'w'+'e'+'avers'='WEAVERS'
(Other definitions for weavers that I've seen before include "They use looms" , "cotton mill employees" .)