Relative calmness ultimately exists (3)
I believe the answer is:
sis
'relative' is the definition.
(short for sister)
'calmness ultimately exists' is the wordplay.
'ultimately' indicates one should take the final letters.
'exists' becomes 'is' ('be' can be a synonym of 'exist'**).
The last letter of 'calmness' is 's'.
's'+'is'='SIS'
(Other definitions for sis that I've seen before include "Secret Intelligence Service (abbreviation)" , "Relative's" , "relative" , "Girl" , "Family member (informal)" .)