Forty days of fasting (4)
I believe the answer is:
lent
'forty days of fasting' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lent that I've seen before include "Christian penitential period" , "Christian penitential season" , "Time of fasting" , "Forty days of penance" , "days before Easter" .)