Calculate; be of the opinion (6)
I believe the answer is:
reckon
This clue is a double definition.
'calculate' is the first definition.
(I know that calculate can be written as reckon)
'be of the opinion' is the second definition.
(I've seen this before)
(Other definitions for reckon that I've seen before include "Believe, suppose, judge" , "Estimate - judge" , "Have as opinion or count up" , "Suppose or calculate" , "Calculate or conclude" .)