Cardinal rejected eating pub grub . . . (6)
I believe the answer is:
dinner
'grub' is the definition.
(I've seen this before)
'cardinal rejected eating pub' is the wordplay.
'cardinal' becomes 'red' (cardinal is a shade of red).
'rejected' shows that the letters should be reversed in order.
'eating' means one lot of letters goes inside another.
'pub' becomes 'inn' (an inn is a type of pub).
'red' back-to-front is 'der'.
'der' enclosing 'inn' is 'DINNER'.
(Other definitions for dinner that I've seen before include "Main meal of day" , "grub" , "Evening meal" , "serving in 20 [MESS] ?" , "Repast" .)