The old man's entertaining celebrity baddies (8)
I believe the answer is:
Here is my best explanation:
'baddies' is the definition.
'the old man's entertaining celebrity' is the subsidiary indication.
'the old man's' becomes 'dads' ('dad' can be a synonym of 'old man').
'entertaining' is an insertion indicator.
'celebrity' becomes 'star' (I've seen this in other clues).
'dads' enclosing 'star' is 'dastards'.
(Other definitions for dastards that I've seen before include "scoundrels" and "Cowards".)