Play piano in class (5)
I believe the answer is:
sport
'play' is the definition.
(I know this)
'piano in class' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'in' is an insertion indicator.
'class' becomes 'sort' (synonyms).
'p' placed into 'sort' is 'SPORT'.
(Other definitions for sport that I've seen before include "Eg, cricket" , "Games" , "Put on" , "Abnormal" , "form of exercise?" .)