Greek character working after piles damaged (7)
I believe the answer is:
epsilon
'greek character' is the definition.
(letter from the Greek alphabet)
'working after piles damaged' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'after' means one lot of letters go next to another.
'damaged' indicates anagramming the letters.
'piles' is an anagram of 'epsil'.
'on' put after 'epsil' is 'EPSILON'.
(Other definitions for epsilon that I've seen before include "One's lip (anag)" , "Lose pin (anag.)" , "Fifth brightest star in a constellation" , "Fifth letter of the Greek alphabet" , "small number mispronounced, almost completely the wrong way round" .)