Slope in garden altered to accommodate one plant finally (8)
I believe the answer is:
gradient
'slope' is the definition.
(I've seen this before)
'garden altered to accommodate one plant finally' is the wordplay.
'altered' is an anagram indicator.
'to accommodate' indicates putting letters inside.
'one' becomes 'I' (Roman numeral).
'finally' says to take the final letters.
The final letter of 'plant' is 't'.
'garden' anagrammed gives 'graden'.
'graden' placed around 'i' is 'gradien'.
'gradien'+'t'='GRADIENT'
'in' is the link.
(Other definitions for gradient that I've seen before include "Bank" , "Level of steepness" , "Slope or incline" , "Rise" , "Degree of slope in a road" .)