Actually concerning a confederate (6)
I believe the answer is:
really
'actually' is the definition.
(synonyms)
'concerning a confederate' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'a confederate' becomes 'ally' ('ally' can be a synonym of 'confederate').
're'+'ally'='REALLY'
(Other definitions for really that I've seen before include "are you sure?" , "Dead" , "Actually, no kidding" , "Do you mean it?" , "In genuine way" .)