Rough calculation (10)
'rough calculation' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for estimate that I've seen before include "Gauge - approximation" , "Make an educated guess" , "rough judgment" , "Calculate approximately" , "damage" .)
(Other definitions for estimation that I've seen before include "Judgment" , "Guess" , "View" , "Reckoning of value" , "Opinion" .)