Competing in pursuit of sides in league is taxing (7)
I believe the answer is:
'taxing' is the definition.
The answer and definition can be both to do with ownership as well as being present participle verbs.
Maybe there's a link between them I don't understand?
'competing in pursuit of sides in league' is the wordplay.
'competing' becomes 'vying' ('vie' can be a synonym of 'compete').
'in pursuit of' means one lot of letters go next to another.
'sides in league' becomes 'le' (I can't explain this - if you can you should give a lot more credence to this answer).
'vying' put after 'le' is 'LEVYING'.
'is' is the link.
Can you help me to learn more?
(Other definitions for levying that I've seen before include "The imposition of taxes", "Imposing (a tax, etc)".)