A rough calculation (8)
I believe the answer is:
estimate
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">a rough calculation</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for estimate that I've seen before include "Make an educated guess" , "Likely amount" , "Tease Tim (anag.)" , "Statement of expected cost" , "Judge the worth of" .)