Agreement number five hundred (3)
I believe the answer is:
nod
'agreement' is the definition.
(a nod indicates agreement)
'number five hundred' is the wordplay.
'number' becomes 'no' (abbreviation).
'five hundred' becomes 'd' (Roman numeral for 500).
'no'+'d'='NOD'
(Other definitions for nod that I've seen before include "Gesture of agreement" , "sign of approval" , "Signal with the head" , "sleepy land" , "Drop the head" .)