Reach an opinion based on reasoning (8)
I believe the answer is:
conclude
'reach an opinion based on reasoning' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for conclude that I've seen before include "Terminate; deduce" , "Bring to a close" , "Finish - deduce" , "Deduce - end" , "Close, end" .)