Grass snake, sort of (5)
I believe the answer is:
sneak
'grass' is the definition.
(thesaurus)
'snake sort of' is the wordplay.
'sort of' is an anagram indicator.
'snake' with letters rearranged gives 'SNEAK'.
(Other definitions for sneak that I've seen before include "In school, tell tales" , "Tale-teller; go furtively" , "Petty informer" , "Go furtively - inform on another child" , "Cringe" .)