I've entered into neat set-up that's simplicity itself (7)
I believe the answer is:
naivete
'set-up that's simplicity itself' is the definition.
I can't judge whether this definition defines the answer.
'i've entered into neat' is the wordplay.
I cannot really see how this works, but
'i've' could be 'ive' and 'ive' is found in the answer.
an anagram of 'neat' is 'nate' which is within the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for naivete that I've seen before include "Ingenuousness" , "Innocence, artlessness" , "State of inexperience (French spelling)" , "Quality of the simple" , "natural simplicity" .)