Application of tax includes queen (8)
I believe the answer is:
exercise
'application' is the definition.
(thesaurus)
'tax includes queen' is the wordplay.
'tax' becomes 'excise' (excise is a kind of tax**).
'includes' means one lot of letters goes inside another.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'excise' placed around 'er' is 'EXERCISE'.
'of' is the link.
(Other definitions for exercise that I've seen before include "Practice session" , "Do it to get fit" , "Task set to practise" , "Exertion to develop endurance or skill" , "Healthy activity" .)