Use fear regularly to cut tax (8)
I believe the answer is:
exercise
'use' is the definition.
('exercise' can be a synonym of 'use')
'fear regularly to cut tax' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'to cut' means one lot of letters goes inside another.
'tax' becomes 'excise' (excise is a kind of tax**).
The alternate letters of 'fear' are 'er'.
'er' put inside 'excise' is 'EXERCISE'.
(Other definitions for exercise that I've seen before include "Physical exertion for keeping fit" , "Task performed to develop skill" , "Task for pupil" , "Exertion to develop endurance or skill" , "problem" .)