Returning ship, poetically, is a national symbol (4)
I believe the answer is:
leek
'a national symbol' is the definition.
(emblem of Wales)
'returning ship' is the wordplay.
'returning' says the letters should be written backwards.
'ship' becomes 'keel' (literary term for a ship).
'keel' back-to-front is 'LEEK'.
'poetically is' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for leek that I've seen before include "Staffordshire town?" , "Vegetable related to onions" , "Onion-like vegetable" , "Vegetable - Welsh emblem" , "Plant related to the onion, pair with potato for soup" .)