Becoming mostly bitter, tucking into a lot of beef (8)
I believe the answer is:
gracious
'becoming' is the definition.
(becoming can mean gracious or decent)
'mostly bitter tucking into a lot of beef' is the wordplay.
'mostly bitter' becomes 'aci' (I can't explain this - if you can you should give a lot more credence to this answer).
'tucking into' means one lot of letters goes inside another.
'a lot of' means to remove the last letter (most of the word but not all of it).
'beef' becomes 'grouse' (synonyms).
'grouse' with its final letter removed is 'grous'.
'aci' inserted into 'grous' is 'GRACIOUS'.
(Other definitions for gracious that I've seen before include "Courteous, kind, pleasant" , "polite expression of surprise" , "alternative to exclamation of grief?" , "Showing qualities associated with good taste and manners" , "Elegant - polite" .)