Adults in classes describing heartless women (5-3)

'adults' is the definition.
(grown-up is a kind of adult)

'classes describing heartless women' is the wordplay.
'classes' becomes 'groups' (grouping is a kind of classing**).
'describing' indicates putting letters inside (describe can mean to form the outline of).
'heartless' means to remove the middle letters.
'women' with its centre taken out is 'wn'.
'groups' enclosing 'wn' is 'GROWN-UPS'.

'in' acts as a link.