Horrid red giant steps up: or down! (8)
I believe the answer is:
gradient
'steps up or down' is the definition.
I can't tell whether this defines the answer.
'horrid red giant' is the wordplay.
'horrid' indicates anagramming the letters.
'red'+'giant'='redgiant'
'redgiant' is an anagram of 'GRADIENT'.
Can you help me to learn more?
(Other definitions for gradient that I've seen before include "Rise" , "Perhaps one in ten" , "Treading (anag) - slope" , "Level of steepness" , "Angle of slope" .)