Beholden to have mixture of gruel and fat (8)
I believe the answer is:
grateful
'beholden' is the definition.
(I've seen this before)
'mixture of gruel and fat' is the wordplay.
'mixture of' indicates an anagram (I've seen this in other clues).
'and' says to put letters next to each other (I've seen this in other clues).
'gruel' after 'fat' is 'fatgruel'.
'fatgruel' with letters rearranged gives 'GRATEFUL'.
'to have' is the link.
(Other definitions for grateful that I've seen before include "quantity of coal" , "Beholden" , "Felt a rug (anag)" , "Thankful, appreciative" , "Showing thankfulness" .)