The collider at Cern (6)
I believe the answer is:
hadron
'the collider at cern' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hadron that I've seen before include "A tiny thing" , "Subatomic particle" , "The Collider at Cern" , "Tiniest thing" , "Large -- Collider" .)