Relate narrative originally supporting pirate at sea (7)
I believe the answer is:
pertain
'relate' is the definition.
(synonyms)
'narrative originally supporting pirate at sea' is the wordplay.
'originally' indicates taking the first letters.
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'at sea' indicates anagramming the letters.
The initial letter of 'narrative' is 'n'.
'pirate' anagrammed gives 'pertai'.
'n' put after 'pertai' is 'PERTAIN'.
(Other definitions for pertain that I've seen before include "Painter inclined to be relevant" , "Be related or applicable (to something)" , "Have relevance" , "Refer" , "Relate (to), apply (to)" .)