Third student learned practice (5)
I believe the answer is:
usage
'practice' is the definition.
(I know that usage is a type of practice)
'third student learned' is the wordplay.
'third student' becomes 'u' (3rd letter of 'student').
'learned' becomes 'sage' (both can mean wise).
'u'+'sage'='USAGE'
(Other definitions for usage that I've seen before include "Custom; practice" , "Way of treating" , "etiquette" , "Terminology - custom" , "Treatment; tradition" .)