Work out tax, impressing English revenue's chief (8)
I believe the answer is:
exercise
'work out' is the definition.
('exercise' can be a synonym of 'work out')
'tax impressing english revenue's chief' is the wordplay.
'tax' becomes 'excise' (excise is a kind of tax**).
'impressing' indicates putting letters inside (impress historically meant to conscript).
'chief' indicates taking the first letters.
The initial letters of 'english revenues' is 'er'.
'excise' enclosing 'er' is 'EXERCISE'.
(Other definitions for exercise that I've seen before include "Physical exertion for keeping fit" , "Physical training or exertion" , "Physical activity done for health and fitness" , "Task performed to develop skill" , "Task set to practise" .)