Moderate power running through damaged meter (6)
I believe the answer is:
temper
'moderate' is the definition.
(I know that moderate can be written as temper)
'power running through damaged meter' is the wordplay.
'power' becomes 'p' (symbol in physics).
'running through' means one lot of letters goes inside another.
'damaged' indicates anagramming the letters.
'meter' is an anagram of 'temer'.
'p' going into 'temer' is 'TEMPER'.
(Other definitions for temper that I've seen before include "Moderate - mood" , "Tantrum" , "Irritability" , "Harden using heat" , "Lose it and lose control" .)