Monstrous error when defending dissolute heirs (7)
I believe the answer is:
ogreish
'monstrous' is the definition.
(I've seen this in another clue)
'error when defending dissolute heirs' is the wordplay.
'error when defending' becomes 'OG' (own goal).
'dissolute' indicates an anagram.
'heirs' with letters rearranged gives 'reish'.
'og'+'reish'='OGREISH'
(Other definitions for ogreish that I've seen before include "Cruel and terrifying" , "Like a cruel monster" , "Looking like a fiend" , "Fearsome" , "Monstrous, misshapen, cruel" .)