Dreadful hour - snored terribly (10)
I believe the answer is:
horrendous
'dreadful' is the definition.
(I know that dreadful can be written as horrendous)
'hour snored terribly' is the wordplay.
'terribly' indicates an anagram.
'hour'+'snored'='hoursnored'
'hoursnored' anagrammed gives 'HORRENDOUS'.
(Other definitions for horrendous that I've seen before include "Absolutely awful" , "Dreadful" , "Ghastly" , "Horrific" , "Criminal" .)