Royal couple caught in corruption overthrown? A nightmare (6)
I believe the answer is:
terror
'a nightmare' is the definition.
(thesaurus)
'royal couple caught in corruption overthrown?' is the wordplay.
'royal couple' means two replacements for 'royal'.
'royal' becomes 'er' (abbreviation for Elizabeth Regina).
'royal' becomes 'r' (abbreviation).
'caught in' indicates putting letters inside.
'corruption' becomes 'rot' (I've seen this in other clues).
'overthrown?' says the letters should be written in reverse.
'rot' reversed gives 'tor'.
'er'+'r'='err'
'err' put inside 'tor' is 'TERROR'.
(Other definitions for terror that I've seen before include "Extreme fear of dread" , "Sheer fright" , "(What causes) panic" , "State of extreme fear" , "Out-of-control child" .)