Fruit dull inside as well (6)
I believe the answer is:
tomato
'fruit' is the definition.
(type of fruit)
'dull inside as well' is the wordplay.
'dull' becomes 'mat' (mat as opposed to glossy).
'inside' means one lot of letters goes inside another.
'as well' becomes 'too' (both can mean 'also').
'mat' put within 'too' is 'TOMATO'.
(Other definitions for tomato that I've seen before include "Fruit-vegetable" , "A motto, oddly, for this fruit" , "Fruit used in salads" , ""Plum" or "beef" fruit?" , "Shiny red fruit" .)