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What is a secondary DNS zone?
The secondary zone — historically the slave — is a read-only copy of the zone, kept current by transfer from the primary.
Records cannot be edited on a secondary. It receives the zone by AXFR after a NOTIFY, or after its refresh timer expires.
A secondary answers queries exactly like the primary. From outside there is no difference, which is the point: any nameserver in the delegation can serve the domain alone.
We can act as secondary for a zone whose primary you keep elsewhere. You publish our nameservers in the delegation and allow transfers to them.
If a secondary answers an old value, look at the serial before anything else — a serial that did not rise is the usual cause.