Particle detector co-creator (6)
I believe the answer is:
geiger
'particle detector co-creator' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for geiger that I've seen before include "He developed a radiation counter" , "Radiation counter inventor" , "German physicist" , "Counter developer for hot spots" , "Counter to measure radiation" .)