Decimal used in some tricky calculation (6)
I believe the answer is:
metric
'decimal' is the definition.
(I've seen this before)
'in some tricky calculation' is the wordplay.
'in' indicates the answer is hidden within the clue.
'METRIC' is hidden within 'some tricky calculation'.
'used' acts as a link.
(Other definitions for metric that I've seen before include "Like some feet" , "Non-Imperial system of measurement" , "Like decimal measuring system" , "System of weights and measures" , "Quantitative" .)