Sparkle of good settled theory now and then (7)
I believe the answer is:
glitter
'sparkle' is the definition.
('glitter' can be a synonym of 'sparkle')
'good settled theory now and then' is the wordplay.
I cannot quite understand how this works, but
'good' could be 'g' (abbreviation) and 'g' is present in the answer.
'settled' could be 'lit' (I've seen this before) and 'lit' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'of' is the link.
Can you help me to learn more?
(Other definitions for glitter that I've seen before include "Twinkle" , "Shine brightly, coldly" , "Flashiness" , "addition to Christmas card" , "Show some sparkle" .)