Fruit characterised by overly dull jams (6)
I believe the answer is:
tomato
'fruit' is the definition.
(type of fruit)
'characterised by overly dull jams' is the wordplay.
I cannot quite understand how this works, but
'by' could be 'at' and 'at' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tomato that I've seen before include "Fruit-vegetable" , "A glossy red fruit eaten as a vegetable" , "Part of a BLT sandwich" , "Shiny red fruit" , "It's also known as a love apple" .)