Concede, about to pass on slope (8)
I believe the answer is:
gradient
'slope' is the definition.
(thesaurus)
'concede about to pass on' is the wordplay.
'concede' becomes 'grant' ('grant' can be a synonym of 'concede').
'about' means one lot of letters goes inside another.
'to pass on' becomes 'die' (I've seen this before).
'grant' going around 'die' is 'GRADIENT'.
(Other definitions for gradient that I've seen before include "Degree of slope in a road" , "Rise" , "Third OT book" , "Inclination" , "Angle of slope" .)