Proves sectarians will be dispersed (10)
I believe the answer is:
ascertains
'proves' is the definition.
The answer and definition can be both related to thinking as well as being verbs in their -s form.
Perhaps they are linked in a way I don't understand?
'sectarians will be dispersed' is the wordplay.
'will be dispersed' indicates anagramming the letters.
'sectarians' with letters rearranged gives 'ASCERTAINS'.
Can you help me to learn more?
(Other definitions for ascertains that I've seen before include "Finds out" , "Discovers" .)