The "A" in AYCE (3)
I believe the answer is:
all
'the a in ayce' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for all that I've seen before include "Total" , "SECOND PART OF 8" , "Every single one" , "See 10 Across" , "'Bless 'em - - ' (song)" .)