Call on a god or spirit (6)
I believe the answer is:
invoke
'call on a god or spirit' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for invoke that I've seen before include "Implore assistance or address in prayer" , "Call in aid" , "Call forth, cite" , "Call on (a deity)" , "Call on a god to do something" .)