Slope needs orderly treading (8)
'slope' is the definition.
(I've seen this before)
'orderly treading' is the wordplay.
'orderly' indicates an anagram.
'treading' anagrammed gives 'GRADIENT'.
'needs' is the link.
(Other definitions for gradient that I've seen before include "Rise", "Treading (anag) - slope", "Third OT book", "Inclination", "Measure of slope".)