Ring a girl to come round soon (4)
I believe the answer is:
anon
'soon' is the definition.
(synonyms)
'ring a girl to come round' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'a girl' becomes 'ann' (woman's name).
'to come round' means one lot of letters goes inside another.
'o' put inside 'ann' is 'ANON'.
(Other definitions for anon that I've seen before include "Shortly; unknown" , "Immediately" , "Of unknown authorship" , "Without known author" , "Soon - nobody knows me" .)