Thankful for fat mixed with gruel (8)
I believe the answer is:
grateful
'thankful' is the definition.
('grateful' can be similar in meaning to 'thankful')
'fat mixed with gruel' is the wordplay.
'mixed with' is an anagram indicator.
'fat' anagrammed with 'gruel' can make 'GRATEFUL'.
'for' acts as a link.
(Other definitions for grateful that I've seen before include "Showing appreciation for kindness" , "Beholden" , "Showing gratitude" , "Felt a rug (anag)" , "Showing thankfulness" .)