American determined to retain ultimate in anonymity, until now (2,3)
I believe the answer is:
as yet
'until now' is the definition.
(I've seen this before)
'american determined to retain ultimate in anonymity' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'determined' becomes 'set' ('set' can be a synonym of 'determined').
'to retain' is an insertion indicator.
'ultimate in' says to take the final letters.
The final letter of 'anonymity' is 'y'.
'set' going around 'y' is 'syet'.
'a'+'syet'='AS YET'
(Other definitions for as yet that I've seen before include "Up to this point in time" , "Up till now" , "Thus far" .)