Most of the mission involved religion (9)
I believe the answer is:
shintoism
'religion' is the definition.
(I know that Shintoism is a type of religion)
'most of the mission involved' is the wordplay.
'most of' means to remove the last letter.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'the' with its last letter removed is 'th'.
'th'+'mission'='thmission'
'thmission' anagrammed gives 'SHINTOISM'.
(Other definitions for shintoism that I've seen before include "Japanese religion" , "foreign religion" .)