Dragon killed by Apollo at Delphi (6)
I believe the answer is:
python
'dragon killed by apollo at delphi' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for python that I've seen before include "crawler" , "Type of snake" , "Large snake that crushes and swallows its prey" , "Large non-venomous snake" , "Heavy-bodied snake" .)