Terribly bad luck: a right good scoundrel (10)
I believe the answer is:
blackguard
'scoundrel' is the definition.
(I know that blackguard is a type of scoundrel)
'terribly bad luck a right good' is the wordplay.
'terribly' indicates an anagram.
'right' becomes 'r' (abbreviation).
'good' becomes 'g' (abbreviation).
'bad'+'luck'+'a'+'r'+'g'='badluckarg'
'badluckarg' anagrammed gives 'BLACKGUARD'.
(Other definitions for blackguard that I've seen before include "Contemptible scoundrel" , "Villain" , "Cad" , "Dishonourable man" , "miscreant" .)