Mysterious nuclear bits in motion (11)
I believe the answer is:
inscrutable
'mysterious' is the definition.
(I know that mysterious can be written as inscrutable)
'nuclear bits in motion' is the wordplay.
'in motion' indicates anagramming the letters (I've seen this in other clues).
'nuclear'+'bits'='nuclearbits'
'nuclearbits' with letters rearranged gives 'INSCRUTABLE'.
(Other definitions for inscrutable that I've seen before include "hard to understand" , "Impossible to understand" , "Cryptic" , "Mysterious" , "Difficult to get" .)