Cor! Menus out for diner? (8)
I believe the answer is:
consumer
'diner?' is the definition.
(diner is a kind of consumer)
'cor menus out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'cor'+'menus'='cormenus'
'cormenus' with letters rearranged gives 'CONSUMER'.
'for' acts as a link.
(Other definitions for consumer that I've seen before include "Person who uses goods or services" , "Person who buys goods or service" , "One buying goods" , "Buyer of goods" , "Client" .)