Use tax to house former monarch (8)
I believe the answer is:
exercise
'use' is the definition.
(exercising is a kind of using)
'tax to house former monarch' is the wordplay.
'tax' becomes 'excise' (excise is a kind of tax).
'to house' is an insertion indicator.
'former monarch' becomes 'ER' (Elizabeth Regina, ie Queen Elizabeth).
'excise' placed around 'er' is 'EXERCISE'.
(Other definitions for exercise that I've seen before include "Physical training or exertion" , "Keep-fit" , "Practice session" , "School task; drill" , "Exertion to develop endurance or skill" .)