About to stuff United, but not now? (4)
I believe the answer is:
once
'not now?' is the definition.
(I've seen this in another clue)
'about to stuff united' is the wordplay.
'about' becomes 'c' (circa).
'to stuff' indicates putting letters inside.
'united' becomes 'one' (as in a phrase like 'we are one').
'c' going inside 'one' is 'ONCE'.
'but' acts as a link.
(Other definitions for once that I've seen before include "On one occasion" , "On a single (or past) occasion" , "in bygone era" , "in days of yore?" , "Previously" .)