Quiet student relaxed, happy (7)
I believe the answer is:
pleased
'happy' is the definition.
(synonyms)
'quiet student relaxed' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'student' becomes 'L' (as in L-plates for learner drivers).
'relaxed' becomes 'eased' (I've seen this before).
'p'+'l'+'eased'='PLEASED'
(Other definitions for pleased that I've seen before include "Content" , "Well satisfied" , "Euphoric" , "Feeling satisfaction" , "Glad" .)