Talk about King Sargon's principal informer (5)
I believe the answer is:
grass
'informer' is the definition.
(thesaurus)
'talk about king sargon's principal' is the wordplay.
'talk' becomes 'gas' (I've seen this before**).
'about' means one lot of letters goes inside another.
'king' becomes 'r' (r is abbreviation for rex).
'principal' suggests taking the first letters.
The initial letter of 'sargon' is 's'.
'gas' going around 'r' is 'gras'.
'gras'+'s'='GRASS'
(Other definitions for grass that I've seen before include "Shopper" , "Ruminants' food" , "German author" , "Inform on someone" , "Plant used for lawns" .)