Bread filling that is dull (5)
I believe the answer is:
inane
'dull' is the definition.
(inane can foolish or uninspired)
'bread filling that is' is the wordplay.
'bread' becomes 'nan' (variant spelling of naan).
'filling' means one lot of letters goes inside another.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'nan' put inside 'ie' is 'INANE'.
(Other definitions for inane that I've seen before include "Lacking sense, silly" , "Really stupid" , "Silly, witless" , "Foolish, fatuous" , "Absurd" .)