Oral examination initially routine in all animal enclosures (7)
I believe the answer is:
Here is my best explanation:
'animal enclosures' is the definition.
'oral examination initially routine in all' is the subsidiary indication.
'oral examination' becomes 'viva' (I've seen this before).
'initially' suggests taking the first letters.
The initial letters of 'routine in all' is 'ria'.
(Other definitions for vivaria that I've seen before include "Tanks, perhaps", "required in all animal study centres" and "animal enclosures".)