Soon as there's a vacancy, a girl is sent round (4)
I believe the answer is:
anon
'soon' is the definition.
(synonyms)
'there's a vacancy a girl is sent round' is the wordplay.
'there's a vacancy' becomes 'o' (I can't explain this - if you can you should give a lot more credence to this answer).
'a girl' becomes 'Ann' (woman's name).
'is sent round' indicates putting letters inside.
'o' inserted inside 'ann' is 'ANON'.
'as' is the link.
Can you help me to learn more?
(Other definitions for anon that I've seen before include "coming after a call" , "Mystery writer" , "Soon; unnamed" , "Soon, in a little while" , "Of unknown authorship" .)