Fruit excessively dull inside (6)
I believe the answer is:
tomato
'fruit' is the definition.
(type of fruit)
'excessively dull inside' is the wordplay.
'excessively' becomes 'too' (to too great a degree).
'dull' becomes 'mat' (mat as opposed to glossy).
'inside' is an insertion indicator.
'too' enclosing 'mat' is 'TOMATO'.
(Other definitions for tomato that I've seen before include "Also called the love apple" , "Fruit used in salads" , "Pulpy fruit eaten as vegetable" , ""Plum" or "beef" fruit?" , "Salad fruit or berry" .)