Devil about to damage one do-gooder (9)
I believe the answer is:
samaritan
'do-gooder' is the definition.
(I've seen this in another clue)
'devil about to damage one' is the wordplay.
'devil' becomes 'satan' (synonyms).
'about' is an insertion indicator.
'to damage' becomes 'mar' (to mar is to spoil or damage).
'one' becomes 'i' (Roman numeral).
'mar'+'i'='mari'
'satan' placed around 'mari' is 'SAMARITAN'.
(Other definitions for samaritan that I've seen before include "Helpful type" , "a helpful sort" , "Charitable person" , "Good man of the Bible" , "good-hearted man" .)