Slice of Roquefort goes with fab black bitter (5)
I believe the answer is:
acerb
'bitter' is the definition.
(synonyms)
'slice of roquefort goes with fab black' is the wordplay.
'slice of roquefort goes' becomes 'r' (I can't justify this - if you can you should believe this answer much more).
'with' means one lot of letters go next to another.
'fab' becomes 'ace' (both can mean excellent).
'black' becomes 'b' (abbreviation as in pencil hardness).
'r' put after 'ace' is 'acer'.
'acer'+'b'='ACERB'
Can you help me to learn more?
(Other definitions for acerb that I've seen before include "tart" , "sharp" , "harsh and bitter" , "Sour, bitter" .)