Tax includes start of extra rate for drill (8)
I believe the answer is:
exercise
'for drill' is the definition.
(synonyms)
'tax includes start of extra rate' is the wordplay.
'tax' becomes 'excise' (excise is a kind of tax).
'includes' is an insertion indicator.
'start of' says to take the initial letters.
The first letters of 'extra rate' is 'er'.
'excise' going around 'er' is 'EXERCISE'.
(Other definitions for exercise that I've seen before include "Train, work out" , "Workout" , "Practice piece of work" , "Physical activity done for health and fitness" , "worry" .)