Diagram when reduced is a worthless thing (3)
I believe the answer is:
fig
'diagram' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'when reduced is a worthless thing' is the wordplay.
'when' becomes 'if'.
'reduced' means to remove the first letter.
'is a worthless thing' becomes 'ig' (I am not sure about this - if you are sure you should believe this answer much more).
'if' with its initial letter removed is 'f'.
'f'+'ig'='FIG'
Can you help me to learn more?
(Other definitions for fig that I've seen before include "Soft pear-shaped fruit" , "Tree whose leaf may cover nudity" , "Sticky fruit" , "Fruit with many seeds" , "Kind of tree in effigy" .)