Enduring horrible revenge, sovereign incarcerated (9)
I believe the answer is:
evergreen
'enduring' is the definition.
(thesaurus)
'horrible revenge sovereign incarcerated' is the wordplay.
'horrible' indicates an anagram.
'sovereign' becomes 'er' (Abbreviation for Elizabeth Regina).
'incarcerated' means one lot of letters goes inside another.
'revenge' is an anagram of 'evgreen'.
'evgreen' enclosing 'er' is 'EVERGREEN'.
(Other definitions for evergreen that I've seen before include "box, perhaps" , "Enduringly popular" , "Laurel for one" , "Timeless" , "Ivy, perhaps" .)