Money demanded for Swallows and Amazons author? Not quite! (6)
I believe the answer is:
'money demanded' is the definition.
'swallows and amazons author? not quite' is the wordplay.
'swallows and amazons author?' becomes 'ransome' (I've seen this before).
'not quite' means to remove the last letter (most but not all of the word).
'ransome' with its final letter taken away is 'RANSOM'.
'for' acts as a link.
(Other definitions for ransom that I've seen before include "Kidnap money" , "A kidnapper demands it" , "Money to get a hostage released" , "Money demand" , "Means to release prisoner" .)