This is a charade - a technique where two or more separate words or word parts are joined together in sequence to form the answer.
Break this into two parts: 'hour' gives you its abbreviation (one letter), and 'dinner time?' refers to a specific time mentioned in a famous play and film title 'Dinner at ___'. Put these together in order.
Answer: HEIGHT | This is a charade of H (abbreviation for 'hour') plus EIGHT ('dinner time?' - referring to the play and film 'Dinner at Eight' by George S Kaufman).