Sign an agreement! (3)
I believe the answer is:
nod
This clue is a double definition.
'sign' is the first definition.
(I have seen 'Sign of assent' mean 'nod' so perhaps 'sign' could also mean 'nod')
'an agreement' is the second definition.
(a nod indicates agreement)
(Other definitions for nod that I've seen before include "Tacit agreement" , "Go to sleep in this land" , "Cain left Eden for the Land of . . ." , "even Homer sometimes does" , "may get gesture of approval" .)