Grass snake's tail moved (5)
I believe the answer is:
sneak
'grass' is the definition.
(thesaurus)
'snake's tail moved' is the wordplay.
'tail' means to remove the last letter (I've seen 'tailed' mean this (to tail something can mean to remove its end)).
'moved' is an anagram indicator.
'snakes' with its last letter taken off is 'snake'.
'snake' is an anagram of 'SNEAK'.
(Other definitions for sneak that I've seen before include "In school, tell tales" , "Snitcher" , "Go furtively - inform on another child" , "Tale-teller; go furtively" , "Cringe" .)