If you are using the Delhi Airport Metro Express (at the New Delhi station), there is a cloakroom just outside the barriers. Bag does not need to be locked. 100INR up to 4 hours. Longer durations also possible (sorry, not sure of price).

If you are using the Delhi Airport Metro Express (at the New Delhi station), there is a cloakroom just outside the barriers. To get here, you cross the from the mezzanine level (between the bottom arrivals and upper departures levels), across the catwalk towards the metro, and walk past the stairs/entrance to the station.

Bag does not need to be locked. I believe it's cash only. The photos show the hourly and flat-rate prices to store luggage (visited February 2019).