Agreement number 500 (3)
I believe the answer is:
nod
'agreement' is the definition.
(a nod indicates agreement)
'number 500' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'500' becomes 'D' (Roman numeral for 500).
'no'+'d'='NOD'
(Other definitions for nod that I've seen before include "Acknowledge by gesture" , "overlook something" , "Drowse" , "Cain was banished to the land of ..." , "Gesture of agreement" .)