Horrendous fine legal (9)
I believe the answer is:
frightful
'horrendous' is the definition.
(thesaurus)
'fine legal' is the wordplay.
'fine' becomes 'f' (abbreviation).
'legal' becomes 'rightful' (as in the rightful owner of something).
'f'+'rightful'='FRIGHTFUL'
(Other definitions for frightful that I've seen before include "from hell" , "very bad" , "alarm" , "Appalling" , "ghastly" .)