Greeting government agency circular (4)
I believe the answer is:
ciao
'greeting' is the definition.
(I've seen this before)
'government agency circular' is the wordplay.
'government agency' becomes 'cia'.
'circular' becomes 'o' (I've seen this in other clues).
'cia'+'o'='CIAO'
(Other definitions for ciao that I've seen before include "Italian's so long" , "Buongiorno o arrivederci" , "Hi - bye" , "Italian cheerio" , "Italian hello or goodbye" .)