Queensberry's third point about Oscar being no shrinking violet (6)
I believe the answer is:
egoist
'no shrinking violet' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'queensberry's third point about oscar' is the wordplay.
'queensberry's third' becomes 'e' (srd letter of 'queensberry').
'point' becomes 'gist' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'gist' going around 'o' is 'goist'.
'e'+'goist'='EGOIST'
'being' is the link.
Can you help me to learn more?
(Other definitions for egoist that I've seen before include "Self-seeker" , "Conceited and self-centred person" , "Conceited person with little regard for others" , "Bighead" , "self-seeking type" .)