Defendant maintaining Republican is loathsome (8)
I believe the answer is:
accursed
'loathsome' is the definition.
(both can mean hated)
'defendant maintaining republican' is the wordplay.
'defendant' becomes 'accused' (synonyms).
'maintaining' indicates putting letters inside.
'republican' becomes 'r' (abbreviation).
'accused' enclosing 'r' is 'ACCURSED'.
'is' is the link.
(Other definitions for accursed that I've seen before include "Hateful" , "Wretched" , "Detestable (colloq.)" , "Goddamn" , "Cue cards (anag) - doomed" .)