Children’s author’s legendary king, free at last after pay-off (6,7)
I believe the answer is:
'children's author's' is the definition.
(author of the Swallows and Amazons books)
'legendary king free at last after pay-off' is the wordplay.
'legendary king' becomes 'arthur' (King Arthur).
'at last' indicates one should take the final letters.
'after' is a charade indicator (letters next to each other).
'pay-off' becomes 'ransom' (ransom money).
The last letter of 'free' is 'e'.
'e' after 'ransom' is 'ransome'.
(Other definitions for arthur ransome that I've seen before include "Swallows and Amazons author" , "Children's author" .)