Assent in Gretna Green (5)
I believe the answer is:
agree
'assent' is the definition.
(I know that assent is a more specific form of agree)
'in gretna green' is the wordplay.
'in' says the answer is hidden in the clue.
'AGREE' is hidden within 'gretna green'.
(Other definitions for agree that I've seen before include "Concur, coincide" , "Shake on it" , "Be of one mind" , "Say yes to a suggestion" , "See eye to eye" .)