Miscalculation (5)

'miscalculation' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for error that I've seen before include "Blunder or mistake" , "Boo-boo" , "Bug" , "Miscarriage of justice" , "Howler" .)
(Another definition for underestimation that I've seen is " deficient appraisal".)