Start with handle for possible conspiracy theorist (5)
I believe the answer is:
crank
'possible conspiracy theorist' is the definition.
I cannot understand how the rest of the clue works.
(Other definitions for crank that I've seen before include "Person of eccentric habits" , "Eccentric - starting handle" , "Person with strange and often aggressive ideas" , "Turn (engine handle); eccentric" , "Turn a handle - weirdo" .)