Work out margins of error in tax (8)
I believe the answer is:
exercise
'work out' is the definition.
(I know that work out can be written as exercise)
'margins of error in tax' is the wordplay.
'margins of' means to remove the middle letters (only the letters on the margins of the word).
'in' indicates putting letters inside.
'tax' becomes 'excise' (excise is a kind of tax).
'error' with its middle removed is 'er'.
'er' put within 'excise' is 'EXERCISE'.
(Other definitions for exercise that I've seen before include "School task; drill" , "Practice session" , "Practice task" , "Use" , "that every prisoner should have" .)