Terribly bad luck: a right good scoundrel (10)
I believe the answer is:
'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).
'badluckarg' anagrammed gives 'BLACKGUARD'.
(Other definitions for blackguard that I've seen before include "Contemptible scoundrel" , "Villain" , "Cad" , "Dishonourable man" , "miscreant" .)