Agree one should include 'sea', as said to editor (6)
I believe the answer is:
accede
'agree' is the definition.
(acceding is a kind of agreeing)
'one should include sea as said to editor' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is present in the answer.
'editor' could be 'ed' (abbreviation) and 'ed' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for accede that I've seen before include "bow" , "Assent (to something)" , "Say yes to request" , "'Acquiesce, defer (6)'" , "Grant or consent" .)