I think 1 hr is very short time to reach DLI from airport.
Only Terminal 3 (used for all International arrivals & AI, Jet airway's domestic arrivals) of Delhi airport is connected by Airport metro express, if you're getting out from Terminal 1 (used by rest domestic airlines), you would be able to take Airport metro only from Aerocity station, which is 15-20 mins away from Terminal 1 building. Again you need to change metro at NDLS station. Overall I don't think you can manage this whole exercise in 1 hr time. So if possible change ur plan.