No longer able to eat lunch's contents (7)
I believe the answer is:
defunct
'no' is the definition.
Both the answer and definition are adjectives. Maybe there's a link between them I don't understand?
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for defunct that I've seen before include "'No longer in force, inactive (7)'" , "No longer operative" , "no longer valid" , "dead and gone" , "Out of commission" .)