If you must use Greater Anglia to get to Cheshunt then you’ll need an off-peak day return from Boundary zone 6 to Cheshunt which is £4.90. It can be purchased from any station ticket office, plus ticket machines if they offer tickets from another station.
If you can restrict yourself to London Overground trains to Cheshunt then your 60+ Oyster card is valid all the way.
You can also buy a ticket from Enfield Lock to Cheshunt for the same price, available as an e-ticket, if you’d rather not visit a ticket office. The train does not have to call at Enfield Lock, only pass it; which in the case of GA services they all do.