Period in which there's universal grief (8)
I believe the answer is:
mourning
'grief' is the definition.
(thesaurus)
'period in which there's universal' is the wordplay.
'period' becomes 'morning' (a period of the day).
'in which there's' means one lot of letters goes inside another.
'universal' becomes 'u'.
'morning' going around 'u' is 'MOURNING'.
(Other definitions for mourning that I've seen before include "wearing black" , "Feeling loss" , "Sorrowing" , "Black clothing" , "Grieving" .)