Health agency auxiliaries initially hold it (4)
I believe the answer is:
whoa
'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'.
'who'+'a'='WHOA'
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" .)