Birds — leaders in East Anglian port (7)
I believe the answer is:
swansea
'leaders in east anglian port' is the definition.
'swansea' can be an answer for 'port' (I've seen this before). I'm unsure of the 'leaders in east anglian' bit.
'birds aeur' is the wordplay.
'birds' becomes 'swans' (swan is a kind of bird).
'aeur' becomes 'ea' (I can't justify this - if you can you should believe this answer much more).
'swans'+'ea'='SWANSEA'
Can you help me to learn more?
(Other definitions for swansea that I've seen before include "Home of the DVLA" , "Welsh port" , "Birthplace of Dylan Thomas" , "Welsh city" , "Second city of Wales" .)