Poser concerning this setter's twisted disguise (9)
I believe the answer is:
pseudonym
'disguise' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'poser concerning this setter's twisted' is the wordplay.
'poser' becomes 'pseud' (I've seen this in another clue).
'concerning' becomes 'on' (both can mean 'in relation to').
'this setter's' becomes 'my' (belonging to the crossword setter).
'twisted' is a reversal indicator.
'my' reversed gives 'ym'.
'pseud'+'on'+'ym'='PSEUDONYM'
Can you help me to learn more?
(Other definitions for pseudonym that I've seen before include "Fictitious identity" , "Goliath, for one" , "in the style of George Eliot?" , "Cinephile, for one" , "Pen name - use my pond (anag)" .)