Arachnid is on crop circles (8)
I believe the answer is:
scorpion
'arachnid' is the definition.
(scorpion is a kind of arachnid)
'is on crop circles' is the wordplay.
'circles' is an anagram indicator.
'is'+'on'+'crop'='isoncrop'
'isoncrop' anagrammed gives 'SCORPION'.
(Other definitions for scorpion that I've seen before include "Venomous creature" , "Venemous creature" , "Kind of arachnid with pincers and stinging tail" , "Venomous stinger" , "Animal with a sting in its tail" .)