Instrument failing to maintain charged particle's congruent state (6)
I believe the answer is:
accord
'instrument' is the definition.
(accord is a kind of instrument)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for accord that I've seen before include "Peace and agreement" , "concert" , "Agreement - conformity" , "Myra -, pianist; Rudolf -, Nazi" , "Harmony and fellowship" .)