The Y in FYI (4)
I believe the answer is:
your
'the y in fyi' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for your that I've seen before include "-- Honour; -- Majesty" , "Belonging to the second person" , "-- Majesty; -- Grace" , "solvers?" , "See 5" .)