Last letters of Guy de Maupassant, until now! (3)
I believe the answer is:
yet
'until now' is the definition.
(I've seen this before)
'last letters of guy de maupassant' is the wordplay.
'last letters of' says to take the final letters.
The final letters of 'guy de maupassant' are 'YET'.
(Other definitions for yet that I've seen before include "To this time, nevertheless" , "Although" , "Up till now" , "Up to this time and continuing" , "in spite of that" .)