Car or a person in it (5)
I believe the answer is:
Here is my best explanation:
'car' is the definition.
(diner is a kind of car)
'a person in it' is the wordplay.
I cannot really see how this works, but
'in' is present in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
'person' could be 'dr' (Dr is an example) and 'dr' is found in the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
'or' acts as a link.
(Other definitions for diner that I've seen before include "Small US restaurant", "one at banquet?", "One taking a meal", "American cafe", "Buffet car", "Small American eatery", "Eater", "US eatery", "Restaurant car" and "Railway carriage serving meals".)