At home one assumes Mum exercises well (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

IN(at home/not out) + A(article for “one”/a single item) contained in ( assumes ) [ SH!(an exclamation requesting someone to be silent/to be mum) + PE(abbrev. for “physical education”/exercises and sports taught in schools) ].