Fancy loading packing area on a slope (8)
I believe the answer is:
diagonal
'on a slope' is the definition.
(I've seen this before)
'fancy loading packing area' is the wordplay.
'fancy' indicates an anagram.
'packing' is an insertion indicator.
'area' becomes 'a' (maths abbreviation).
'loading' anagrammed gives 'diagonl'.
'diagonl' going around 'a' is 'DIAGONAL'.
(Other definitions for diagonal that I've seen before include "Joining non-adjacent corners" , "Corner-to-corner line" , "Line from one corner to the opposite corner" , "A straight line joining two opposite corners" , "Line from one corner to the fartherst one" .)