Fresh mindset supporting game's first principles (9)
I believe the answer is:
rudiments
'first principles' is the definition.
('rudiment' can be a synonym of 'first principle')
'fresh mindset supporting game's' is the wordplay.
'fresh' indicates anagramming the letters.
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'game' becomes 'ru' (rugby union).
'mindset' with letters rearranged gives 'diments'.
'diments' put after 'ru' is 'RUDIMENTS'.
(Other definitions for rudiments that I've seen before include "Basics of any subject" , "basic principles" .)