Working class in good heart (2,4)
I believe the answer is:
on form
'good heart' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'working class' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'class' becomes 'form class' (form class is a kind of class).
'on'+'form'='ON FORM'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for on form that I've seen before include "At the top of one's game" , "Fit" , "Playing well" .)