King enthralled by quirky, retro horror (6)
I believe the answer is:
terror
'horror' is the definition.
(thesaurus)
'king enthralled by quirky retro' is the wordplay.
'king' becomes 'R' (abbreviation of Latin rex).
'enthralled by' means one lot of letters goes inside another ('enthral' can mean 'capture').
'quirky' indicates an anagram.
'retro' is an anagram of 'teror'.
'r' put within 'teror' is 'TERROR'.
(Other definitions for terror that I've seen before include "Fear - mischievous child" , "Mischief-maker; dread" , "Fear - now being warred on" , "State of extreme fright" , "Great fear" .)