Refusal to accept knights, say, stifling authority (5,5)
I believe the answer is:
nanny state
'stifling authority' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'refusal to accept knights say' is the wordplay.
'refusal' becomes 'nay' (archaic word for 'no').
'to accept' means one lot of letters goes inside another.
'knights' becomes 'NN' (the chess abbreviation for knight written twice).
'say' becomes 'state' (synonyms).
'nay' going around 'nn' is 'nanny'.
'nanny'+'state'='NANNY STATE'
Can you help me to learn more?
(Other definitions for nanny state that I've seen before include "Overprotective government deplored by Thatcher" , "Interfering government" , "Interfering authorities" , "Undue government interference with personal choice?" .)