Confuse reigning monarch in endless peril (7)
I believe the answer is:
derange
'confuse' is the definition.
(thesaurus)
'reigning monarch in endless peril' is the wordplay.
'reigning monarch' becomes 'ER' (Elizabeth Regina ie Queen Elizabeth).
'in' is an insertion indicator.
'endless' means to remove the last letter.
'peril' becomes 'danger' (synonyms).
'danger' with its last letter taken away is 'dange'.
'er' inserted inside 'dange' is 'DERANGE'.
(Other definitions for derange that I've seen before include "Upset" , "Unhinge" , "Make mad" , "Madden" , "Throw into disorder" .)