To me, an unreasonable grouse is awful (8)
I believe the answer is:
gruesome
'awful' is the definition.
(thesaurus)
'to me an unreasonable grouse' is the wordplay.
I cannot really see how this works, but
'me' is within the answer.
an anagram of 'grouse' is 'grueso' which is within the answer.
No letters remain.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for gruesome that I've seen before include "Horrifying and revolting" , "Repulsive, horrific" , "Repugnant" , "'Ghastly, grisly (8)'" , "'Grisly, macabre (8)'" .)