Run down gradient exploding with energy (9)
I believe the answer is:
denigrate
'run down' is the definition.
(I've seen this before)
'gradient exploding with energy' is the wordplay.
'exploding with' is an anagram indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'gradient' anagrammed with 'e' makes 'DENIGRATE'.
(Other definitions for denigrate that I've seen before include "Criticise, belittle" , "Blacken" , "Bad-mouth" , "put down" , "Run down" .)