Birds — leaders in East Anglian port (7)
I believe the answer is:
swansea
'port' is the definition.
(I know that Swansea is a port)
'birds leaders in east anglian' is the wordplay.
'birds' becomes 'swans' (swan is a kind of bird).
'leaders in' suggests taking the first letters.
The first letters of 'east anglian' is 'ea'.
'swans'+'ea'='SWANSEA'
(Other definitions for swansea that I've seen before include "Welsh city" , "Second city of Wales" , "South Wales port" , "coastal location" , "Ecstasy, a win! (anag)" .)