Ran an experiment on (6)
I believe the answer is:
tested
'ran an experiment on' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tested that I've seen before include "Tried or examined" , "Probed or analysed" , "Subjected to examination or trial" , "Measured for quality or reliability" , "Put to the proof" .)