Talk idly about King Sargon's principal informer (5)
I believe the answer is:
grass
'informer' is the definition.
(grass can mean an informant)
'talk idly about king sargon's principal' is the wordplay.
'talk idly' becomes 'gas' (I've seen this in another clue).
'about' is an insertion indicator.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'principal' indicates taking the first letters (principal letter).
The initial letter of 'sargons' is 's'.
'gas' enclosing 'r' is 'gras'.
'gras'+'s'='GRASS'
(Other definitions for grass that I've seen before include "shop" , "Split - pot - plant" , "Vegetation; informer" , "German author" , "Drug" .)