Brief note about contents of big, fat trifles (8)
I believe the answer is:
minutiae
'trifles' is the definition.
(I've seen this before)
'brief note about contents of big fat' is the wordplay.
'brief note' becomes 'minute' (I've seen this before).
'about' is an insertion indicator.
'contents of big fat' becomes 'ia' (I can't explain this - if you can you should believe this answer much more).
'minute' placed around 'ia' is 'MINUTIAE'.
Can you help me to learn more?
(Other definitions for minutiae that I've seen before include "Detailed particulars" , "Tiny or trivial details" , "Trivia, very small details" , "trifles" , "Precise details" .)