Nigerian seaport (5)
I believe the answer is:
lagos
'nigerian seaport' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lagos that I've seen before include "Chief port and former capital of Nigeria" , "Former Nigerian capital" , "African port" , "Formerly capital of Nigeria" , "Largest city and once capital of Nigeria" .)