Reckon, compute (9)
I believe the answer is:
calculate
This clue is a double definition.
'reckon' is the first definition.
(synonyms)
'compute' is the second definition.
(synonyms)
(Other definitions for calculate that I've seen before include "'Work out, compute (9)'" , "Compute or reckon" , "Work out mathematically" , "Solve by mathematical procedure" , "Think about" .)