Not right upstairs, stagger going up (4)
I believe the answer is:
nuts
'not right upstairs' is the definition.
(both can informally mean mad)
'stagger going up' is the wordplay.
'stagger' becomes 'stun' (both can mean to shock).
'going up' shows that the letters should be reversed in order.
'stun' backwards is 'NUTS'.
(Other definitions for nuts that I've seen before include "Hard-shelled fruits or seeds" , "Mast - mad" , "out to lunch" , "Heads - fans - fruit" , "balls" .)