In time, big upset is essential (8)
I believe the answer is:
integral
'essential' is the definition.
(I've seen this before)
'in time big upset' is the wordplay.
'time' becomes 't' (abbreviation).
'big' becomes 'large' (synonyms).
'upset' shows that the letters should be reversed in order.
'large' in reverse letter order is 'egral'.
'in'+'t'+'egral'='INTEGRAL'
'is' is the link.
(Other definitions for integral that I've seen before include "Fundamental; function" , "Maths function; entire" , "Altering so as to be necessary to the whole" , "Unimpaired" , "Large tin (anag.)" .)