Admitting home's completely out of cups — that's stupid! (9)
I believe the answer is:
brainless
'stupid' is the definition.
('brainless' can be a synonym of 'stupid')
'admitting home's completely out of cups' is the wordplay.
'admitting' means one lot of letters goes inside another.
'home' becomes 'in' ('he's home' can mean 'he's in').
'completely out of cups' becomes 'braless' (a bra is made up of two cups).
'in' put within 'braless' is 'BRAINLESS'.
'that's' acts as a link.
(Other definitions for brainless that I've seen before include "Silly" , "dim" , "Devoid of intelligence" , "Stupid, foolish" .)