At home, one assumes Mum exercises well (2,5)

'well' is the definition.
I know nothing about this answer so I can't judge whether this works.

'at home one assumes mum exercises' is the wordplay.
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'one' becomes 'a' (a thing is one thing).
'assumes' means one lot of letters goes inside another (assume can mean to take on).
'mum' becomes 'sh' (both can be used to tell someone to be quiet).
'exercises' becomes 'pe' (Physical Education class).
'sh'+'pe'='shpe'
'a' placed inside 'shpe' is 'shape'.
'in'+'shape'='IN SHAPE'

Can you help me to learn more?