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' says the answer is hidden in 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" .)