A setter is confused about one exposition (8)
I believe the answer is:
treatise
'exposition' is the definition.
(I've seen this before)
'a setter is confused about one' is the wordplay.
'is confused' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'one' becomes 'I' (Roman numeral).
'a'+'setter'='asetter'
'asetter' anagrammed gives 'treatse'.
'treatse' going around 'i' is 'TREATISE'.
(Other definitions for treatise that I've seen before include "Essay on theme" , "Disquisition" , "Written study" , "Learned work on particular subject" , "Thesis" .)