Seine capital (5)
I believe the answer is:
paris
'seine capital' is the definition.
'Paris' can be an answer for 'capital' (Paris is the capital of France). I also know that 'seine' relates to this answer.
This is the entire clue.
(Other definitions for paris that I've seen before include "Trojan prince who abducted Helen" , "investment resulted from his action" , "Prince who abducted Helen, starting a long war (see 8)" , "English chronicler" , "He abducted Helen of Troy" .)