Itemised a backside, indeed! (8)
I believe the answer is:
detailed
'itemised' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for detailed that I've seen before include "Meticulous" , "Particular" , "supplying full particulars" , "Giving full particulars" , "Elaborate" .)