Perhaps King Charles's plausible story about a knight (7)
I believe the answer is:
spaniel
'perhaps king charles's plausible' is the definition.
I can't tell whether this defines the answer.
'story about a knight' is the wordplay.
'story' becomes 'spiel' (I am not sure about this - if you are sure you should believe this answer much more).
'about' is an insertion indicator.
'knight' becomes 'n' (chess abbreviation).
'a'+'n'='an'
'spiel' placed around 'an' is 'SPANIEL'.
Can you help me to learn more?
(Other definitions for spaniel that I've seen before include "Dog found among the alpines" , "Type of dog" , "Dog with drooping ears" , "Droopy-eared dog" , "[HOUND]" .)