Consider you can do sums (6)
I believe the answer is:
reckon
'consider' is the definition.
(synonyms)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for reckon that I've seen before include "Suppose or calculate" , "what people who count will do" , "Believe, suppose, judge" , "Consider or calculate" , "Calculate, compute or estimate" .)