Bird going to main Welsh port ... (7)
I believe the answer is:
swansea
'welsh port' is the definition.
(I've seen this before)
'bird going to main' is the wordplay.
'bird' becomes 'swan' (swan is a kind of bird).
'going to' means one lot of letters go next to another.
'main' becomes 'sea' (the main is an old name for the sea).
'swan'+'sea'='SWANSEA'
(Other definitions for swansea that I've seen before include "Birthplace of Dylan Thomas" , "South Wales port" , "Welsh port" , "Welsh city" , "South Wales city" .)