Playing well in this class (4)
I believe the answer is:
form
I believe this clue is a double definition.
'playing well' is the first definition.
(as in a sportsperson who is playing 'on form')
'this class' is the second definition.
(synonyms)
'in' acts as a link.
(Other definitions for form that I've seen before include "Printed document with spaces to write" , "Part of school" , "Printed document with blank spaces for information to be inserted" , "Class in a school, or the shape of something" , "Visible shape, class" .)