Agreement in every essential (3)
I believe the answer is:
yes
'agreement' is the definition.
(saying yes to something)
'in every essential' is the wordplay.
'in' indicates the answer is hidden within the clue.
'YES' is hidden in the letters of 'every essential'.
(Other definitions for yes that I've seen before include "OK" , "Oui" , "Positive reply" , "I agree" , "Absolutely" .)