Indifferent to a quiet sporty student being left out (9)
I believe the answer is:
apathetic
'indifferent' is the definition.
(synonyms)
'a quiet sporty student being left out' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'sporty' becomes 'athletic' (I've seen this before).
'student' becomes 'L' (as in L-plates for learner drivers).
'being left out' indicates named letters should be taken away.
'athletic' with 'l' taken out is 'athetic'.
'a'+'p'+'athetic'='APATHETIC'
'to' acts as a link.
(Other definitions for apathetic that I've seen before include "Uncaring, uninterested" , "disinclined to act" , "Impassive" , "cool?" , "I hate pact, being disintersted" .)