Doing our best about a source of damage (8)
I believe the answer is:
saboteur
'a source of damage' is the definition.
(source of sabotage)
'doing our best about' is the wordplay.
'doing' indicates anagramming the letters (doing can mean solving or working out).
'about' becomes 'a' (abbreviation in Chambers).
'our'+'best'+'a'='ourbesta'
'ourbesta' is an anagram of 'SABOTEUR'.
(Other definitions for saboteur that I've seen before include "One who disrupts or damages by design" , "Rue boast about one who commits industrial espionage" , "Vandal" , "Underminer, wrecker" , "One who causes deliberate damage to plants or machinery" .)