Oral examination initially routine in all animal enclosures (7)
I believe the answer is:
vivaria
'animal enclosures' is the definition.
(I've seen this before)
'oral examination initially routine in all' is the wordplay.
'oral examination' becomes 'viva' (I've seen this before).
'initially' suggests taking the first letters.
The first letters of 'routine in all' is 'ria'.
'viva'+'ria'='VIVARIA'
(Other definitions for vivaria that I've seen before include "Tanks, perhaps" , "required in all animal study centres" , "animal enclosures" .)