Calculate (6)
'calculate' is the definition.
(I know that reckon is a more specific form of approximate)
This is the entire clue.
(Other definitions for reckon that I've seen before include "Consider or calculate" , "Calculate, compute or estimate" , "what people who count will do" , "Think - calculate" , "Calculate, be of the opinion" .)
'calculate' is the definition.
(I know that calculate can be written as compute)
This is all the clue.
(Other definitions for compute that I've seen before include "Reckon, calculate" , "Calculate, maybe electronically" , "Work out" .)