At once, once (4)
I believe the answer is:
anon
'at once once' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for anon that I've seen before include "Soon - nobody knows me" , "Soon, precious stone" , "Some time soon" , "Of unknown identity, in short" , "At another time, of old" .)