Series about former king produces discomfiture (7)
I believe the answer is:
chagrin
'discomfiture' is the definition.
(I've seen this before)
'series about former king' is the wordplay.
'series' becomes 'chain' (chain is a kind of series).
'about' is an insertion indicator.
'former king' becomes 'gr' (short for Georgius Rex, King George in Latin).
'chain' going around 'gr' is 'CHAGRIN'.
'produces' acts as a link.
(Other definitions for chagrin that I've seen before include "Arching with annoyance and embarrassment" , "Shame at one's failure" , "irritation?" , "Shame at having failed" , "Feeling of annoyance and disappointment" .)