Completely throwing HRH — too ugly! (10)
I believe the answer is:
thoroughly
'completely' is the definition.
(thesaurus)
'throwing hrh too ugly' is the wordplay.
'throwing' indicates anagramming the letters.
'hrh'+'too'+'ugly'='hrhtoougly'
'hrhtoougly' is an anagram of 'THOROUGHLY'.
(Other definitions for thoroughly that I've seen before include "Quite" , "Completely" , "More than a bit" , "Really" , "Musical with Julie Andrews" .)