Basic principles of game obscure -- sent off! (9)
I believe the answer is:
rudiments
'basic principles' is the definition.
(rudiment is a kind of basic principle)
'game obscure sent off' is the wordplay.
'game' becomes 'ru' (rugby union).
'obscure' becomes 'dim' (I've seen this before).
'off' indicates an anagram.
'sent' anagrammed gives 'ents'.
'ru'+'dim'+'ents'='RUDIMENTS'
'of' is the link.
(Other definitions for rudiments that I've seen before include "basic principles" , "Basics of any subject" .)