Health agency auxiliaries initially hold it (4)
I believe the answer is:
'hold it' is the definition.
Both the definition and answer are interjections.
Perhaps you can see an association between them that I don't see?
'health agency auxiliaries initially' is the wordplay.
'health agency' becomes 'who' (I can't justify this - if you can you should believe this answer much more).
'initially' suggests taking the first letters.
The initial letter of 'auxiliaries' is 'a'.
Can you help me to learn more?
(Other definitions for whoa that I've seen before include "Command to horse to stop or stand still" , "Cry for horses to stop" , "Said to stop a horse" , "Opposite of "Giddy-up!"" , "Order (horse) to stop" .)